[Mac] [Reland] On the Touch Bar, use individual back/forward buttons instead of semented control.

This relands the original change, fixing a use-after-free when
BrowserWindowDefaultTouchBar outlives its profile.

Original change's description:

> This is a design tweak which also enables the back and forward buttons
> to be placed separately. Existing customized Touch Bars which contain
> the grouped item still work; see the code comment for details.
>
> I used Cocoa bindings for the buttons' enabled states instead of holding
> pointers to them because I initially thought that the Touch Bar might allow the
> group and individual buttons to coexist. Turns out no, but I kept the KVO
> implementation because I like it.
>
> It also includes a couple of other changes that came up while writing
> this patch and during code review:
>
> - The unit tests no longer look for a hard-coded list of items in a
>   specific order. Instead, they verify that BrowserWindowDefaultTouchBar
>   can create the items it claims to be able to create, and that no
>   known-valid identifiers have stopped working.
>
> - BrowserWindowDefaultTouchBar returns nil if it doesn't know about an
>   identifier, instead of returning an empty Touch Bar item.
>
> Bug: 937935
> Change-Id: I8cdc6347bc667b0a24fa24fe2c6a56661747ac98
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534238
> Commit-Queue: Sidney San Martín <sdy@chromium.org>
> Reviewed-by: Leonard Grey <lgrey@chromium.org>
> Auto-Submit: Sidney San Martín <sdy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#649899}

Bug: 937935, 945772
Change-Id: I77bf9b6d14bef875f271f8fbd4468e6cdc30434e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1568846
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Leonard Grey <lgrey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651851}

chrome/browser/ui/cocoa/touchbar/browser_window_default_touch_bar.h

@@ -14,10 +14,6 @@ class BookmarkTabHelperObserver;
 class Browser;
 @class BrowserWindowTouchBarController;
 
-namespace content {
-class WebContents;
-}
-
 // Provides a default touch bar for the browser window. This class implements
 // the NSTouchBarDelegate and handles the items in the touch bar.
 API_AVAILABLE(macos(10.12.2))
@@ -29,18 +25,18 @@ API_AVAILABLE(macos(10.12.2))
 // True if the current page is starred. Used by star touch bar button.
 @property(nonatomic, assign) BOOL isStarred;
 
-// Designated initializer.
-- (instancetype)initWithBrowser:(Browser*)browser
-                     controller:(BrowserWindowTouchBarController*)controller;
+// True if the back button is enabled.
+@property(nonatomic, assign) BOOL canGoBack;
 
-// Creates and returns a touch bar for the browser window.
-- (NSTouchBar*)makeTouchBar;
+// True if the forward button is enabled.
+@property(nonatomic, assign) BOOL canGoForward;
+
+@property(nonatomic, assign) BrowserWindowTouchBarController* controller;
 
-- (void)updateWebContents:(content::WebContents*)contents;
+@property(nonatomic) Browser* browser;
 
-// Updates the back/forward button. Called when creating the touch bar or when
-// the back and forward commands have changed.
-- (void)updateBackForwardControl;
+// Creates and returns a touch bar for the browser window.
+- (NSTouchBar*)makeTouchBar;
 
 - (BrowserWindowTouchBarController*)controller;
 
@@ -49,6 +45,11 @@ API_AVAILABLE(macos(10.12.2))
 // Private methods exposed for testing.
 @interface BrowserWindowDefaultTouchBar (ExposedForTesting)
 
+@property(readonly, class) NSString* reloadOrStopItemIdentifier;
+@property(readonly, class) NSString* backItemIdentifier;
+@property(readonly, class) NSString* forwardItemIdentifier;
+@property(readonly, class) NSString* fullscreenOriginItemIdentifier;
+
 // Updates the reload/stop button. Called when creating the touch bar or the
 // page load state has been updated.
 - (void)updateReloadStopButton;
@@ -56,10 +57,6 @@ API_AVAILABLE(macos(10.12.2))
 // Returns the reload/stop button on the touch bar. Creates it if it's null.
 - (NSButton*)reloadStopButton;
 
-// Returns the back/forward segmented control on the touch bar. Creates it if
-// it's null.
-- (NSSegmentedControl*)backForwardControl;
-
 // Returns the bridge object that BrowserWindowDefaultTouchBar uses to receive
 // notifications.
 - (BookmarkTabHelperObserver*)bookmarkTabObserver;

chrome/browser/ui/cocoa/touchbar/browser_window_touch_bar_controller.h

@@ -32,9 +32,6 @@ API_AVAILABLE(macos(10.12.2))
 // nil.
 - (void)invalidateTouchBar;
 
-- (void)updateWebContents:(content::WebContents*)contents;
-
-- (content::WebContents*)webContents;
 @end
 
 @interface BrowserWindowTouchBarController (ExposedForTesting)

chrome/browser/ui/cocoa/touchbar/browser_window_touch_bar_controller.mm

@@ -19,66 +19,9 @@
 #include "content/public/browser/web_contents_observer.h"
 #import "ui/base/cocoa/touch_bar_util.h"
 
-class API_AVAILABLE(macos(10.12.2)) WebContentsNotificationBridge
-    : public TabStripModelObserver,
-      public content::WebContentsObserver {
- public:
-  WebContentsNotificationBridge(BrowserWindowTouchBarController* owner,
-                                Browser* browser)
-      : owner_(owner), browser_(browser), contents_(nullptr) {
-    TabStripModel* model = browser_->tab_strip_model();
-    DCHECK(model);
-    model->AddObserver(this);
-
-    UpdateWebContents(model->GetActiveWebContents());
-  }
-
-  ~WebContentsNotificationBridge() override {
-    TabStripModel* model = browser_->tab_strip_model();
-    if (model)
-      model->RemoveObserver(this);
-  }
-
-  void UpdateWebContents(content::WebContents* new_contents) {
-    contents_ = new_contents;
-    Observe(contents_);
-
-    [owner_ updateWebContents:contents_];
-  }
-
-  // TabStripModelObserver:
-  void OnTabStripModelChanged(
-      TabStripModel* tab_strip_model,
-      const TabStripModelChange& change,
-      const TabStripSelectionChange& selection) override {
-    if (tab_strip_model->empty() || !selection.active_tab_changed())
-      return;
-
-    UpdateWebContents(selection.new_contents);
-    contents_ = selection.new_contents;
-  }
-
-  content::WebContents* contents() const { return contents_; }
-
- protected:
-  // WebContentsObserver:
-  void WebContentsDestroyed() override {
-    // Clean up if the web contents is being destroyed.
-    UpdateWebContents(nullptr);
-  }
-
- private:
-  BrowserWindowTouchBarController* owner_;  // Weak.
-  Browser* browser_;                        // Weak.
-  content::WebContents* contents_;          // Weak.
-};
-
 @interface BrowserWindowTouchBarController () {
   NSWindow* window_;  // Weak.
 
-  // Used to receive and handle notifications.
-  std::unique_ptr<WebContentsNotificationBridge> notificationBridge_;
-
   base::scoped_nsobject<BrowserWindowDefaultTouchBar> defaultTouchBar_;
 
   base::scoped_nsobject<WebTextfieldTouchBarController> webTextfieldTouchBar_;
@@ -92,12 +35,9 @@ class API_AVAILABLE(macos(10.12.2)) WebContentsNotificationBridge
     DCHECK(browser);
     window_ = window;
 
-    notificationBridge_ =
-        std::make_unique<WebContentsNotificationBridge>(self, browser);
-
-    defaultTouchBar_.reset([[BrowserWindowDefaultTouchBar alloc]
-        initWithBrowser:browser
-             controller:self]);
+    defaultTouchBar_.reset([[BrowserWindowDefaultTouchBar alloc] init]);
+    defaultTouchBar_.get().controller = self;
+    defaultTouchBar_.get().browser = browser;
     webTextfieldTouchBar_.reset(
         [[WebTextfieldTouchBarController alloc] initWithController:self]);
   }
@@ -105,6 +45,11 @@ class API_AVAILABLE(macos(10.12.2)) WebContentsNotificationBridge
   return self;
 }
 
+- (void)dealloc {
+  defaultTouchBar_.get().browser = nullptr;
+  [super dealloc];
+}
+
 - (void)invalidateTouchBar {
   DCHECK([window_ respondsToSelector:@selector(setTouchBar:)]);
   [window_ performSelector:@selector(setTouchBar:) withObject:nil];
@@ -118,15 +63,6 @@ class API_AVAILABLE(macos(10.12.2)) WebContentsNotificationBridge
   return [defaultTouchBar_ makeTouchBar];
 }
 
-- (void)updateWebContents:(content::WebContents*)contents {
-  [defaultTouchBar_ updateWebContents:contents];
-  [self invalidateTouchBar];
-}
-
-- (content::WebContents*)webContents {
-  return notificationBridge_->web_contents();
-}
-
 @end
 
 @implementation BrowserWindowTouchBarController (ExposedForTesting)