[ios clean] Fix crashes.

This CL fixes a couple of common crashes in CSChromium.

- Entering a URL on the NTP caused a crash because the webState wasn't enabled
when it was navigating. This is fixed by setting webUsageEnabled during
-loadGURLFromLocationBar:transition:

- Tapping some buttons in the toolbar would cause a crash in the tab container
VC's -canPerformAction:withSender:, which was creating an array with objects
that might be nil. This method was only needed to plumb responder chain events
into contained view controllers, which has long since been replaced by 
dispatchers.

Also, per the ObjC style guide, the exception raised in -subviewConstraints is
replaced with a NOTREACHED().

Bug: 
Change-Id: Ic22bc6decfde97530434c95f318b72cf637aaf24
Reviewed-on: https://chromium-review.googlesource.com/594947Reviewed-by: Ed Chin <edchin@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491949}

ios/clean/chrome/browser/ui/omnibox/location_bar_mediator.mm

@@ -86,6 +86,7 @@
                      transition:(ui::PageTransition)transition {
   web::WebState* webState = _webStateList->GetActiveWebState();
   DCHECK(webState);
+  webState->SetWebUsageEnabled(true);
 
   if (url.SchemeIs(url::kJavaScriptScheme)) {
     // TODO(crbug.com/708341): Percent-unescape the url content first.

ios/clean/chrome/browser/ui/tab/BUILD.gn

@@ -39,6 +39,7 @@ source_set("tab_ui") {
     "tab_container_view_controller.mm",
   ]
   deps = [
+    "//base",
     "//ios/clean/chrome/browser/ui",
     "//ios/clean/chrome/browser/ui/transitions",
     "//ios/clean/chrome/browser/ui/transitions/animators",

ios/clean/chrome/browser/ui/tab/tab_container_view_controller.mm

@@ -4,6 +4,7 @@
 
 #import "ios/clean/chrome/browser/ui/tab/tab_container_view_controller.h"
 
+#import "base/logging.h"
 #import "ios/clean/chrome/browser/ui/transitions/animators/swap_from_above_animator.h"
 #import "ios/clean/chrome/browser/ui/transitions/containment_transition_context.h"
 #import "ios/clean/chrome/browser/ui/transitions/containment_transitioning_delegate.h"
@@ -31,10 +32,6 @@ CGFloat kTabStripHeight = 120.0f;
 @property(nonatomic, strong) NSLayoutConstraint* tabStripHeightConstraint;
 @property(nonatomic, strong) NSLayoutConstraint* toolbarHeightConstraint;
 
-// Cache for forwarding methods to child view controllers.
-@property(nonatomic, assign) SEL actionToForward;
-@property(nonatomic, weak) UIResponder* forwardingTarget;
-
 // Abstract base method for subclasses to implement.
 // Returns constraints for tabStrip, toolbar, and content subviews.
 - (Constraints*)subviewConstraints;
@@ -54,8 +51,6 @@ CGFloat kTabStripHeight = 120.0f;
 @synthesize contentView = _contentView;
 @synthesize tabStripHeightConstraint = _tabStripHeightConstraint;
 @synthesize toolbarHeightConstraint = _toolbarHeightConstraint;
-@synthesize actionToForward = _actionToForward;
-@synthesize forwardingTarget = _forwardingTarget;
 @synthesize containmentTransitioningDelegate =
     _containmentTransitioningDelegate;
 
@@ -225,33 +220,6 @@ CGFloat kTabStripHeight = 120.0f;
   return CGRectNull;
 }
 
-#pragma mark - UIResponder
-
-// Before forwarding actions up the responder chain, give both contained
-// view controllers a chance to handle them.
-- (BOOL)canPerformAction:(SEL)action withSender:(id)sender {
-  self.actionToForward = nullptr;
-  self.forwardingTarget = nil;
-  for (UIResponder* responder in
-       @[ self.contentViewController, self.toolbarViewController ]) {
-    if ([responder canPerformAction:action withSender:sender]) {
-      self.actionToForward = action;
-      self.forwardingTarget = responder;
-      return YES;
-    }
-  }
-  return [super canPerformAction:action withSender:sender];
-}
-
-#pragma mark - NSObject method forwarding
-
-- (id)forwardingTargetForSelector:(SEL)aSelector {
-  if (aSelector == self.actionToForward) {
-    return self.forwardingTarget;
-  }
-  return nil;
-}
-
 #pragma mark - Tab Strip actions.
 
 - (void)hideTabStrip:(id)sender {
@@ -261,9 +229,7 @@ CGFloat kTabStripHeight = 120.0f;
 #pragma mark - Abstract methods to be overriden by subclass
 
 - (Constraints*)subviewConstraints {
-  [NSException
-       raise:NSInternalInconsistencyException
-      format:@"You must override %@ in a subclass", NSStringFromSelector(_cmd)];
+  NOTREACHED() << "You must override -subviewConstraints in a subclass";
   return nil;
 }