Gesture Nav: Protect against the case when navigation completes as soon as it is requested.

If DidStopLoading is called on WebContentsObserver immeditely after navigation
is requested, then window_ in could be released via StopObservingIfDone as soon
as GoForward/GoBack is called

OerscrollNavigationOverlay::OnOverscrollCompleted requests navigaions by calling
GoForward/GoBack on the controller. If these navigations immeditely trigger
DidStopLoading, main_window->SetTransform would segfault.
Protect against this case by setting up the transforms and window positioning
before doing navigation.

BUG=623620

Review-Url: https://codereview.chromium.org/2102283002
Cr-Commit-Position: refs/heads/master@{#403186}

content/browser/web_contents/aura/overscroll_navigation_overlay.cc

@@ -223,6 +223,14 @@ void OverscrollNavigationOverlay::OnOverscrollCompleted(
     return;
   }
 
+  main_window->SetTransform(gfx::Transform());
+  window_ = std::move(window);
+  // Make sure the window is in its default position.
+  window_->SetBounds(gfx::Rect(web_contents_window_->bounds().size()));
+  window_->SetTransform(gfx::Transform());
+  // Make sure the overlay window is on top.
+  web_contents_window_->StackChildAtTop(window_.get());
+
   // Make sure we can navigate first, as other factors can trigger a navigation
   // during an overscroll gesture and navigating without history produces a
   // crash.
@@ -247,13 +255,6 @@ void OverscrollNavigationOverlay::OnOverscrollCompleted(
     StartObserving();
   }
 
-  main_window->SetTransform(gfx::Transform());
-  window_ = std::move(window);
-  // Make sure the window is in its default position.
-  window_->SetBounds(gfx::Rect(web_contents_window_->bounds().size()));
-  window_->SetTransform(gfx::Transform());
-  // Make sure the overlay window is on top.
-  web_contents_window_->StackChildAtTop(window_.get());
   direction_ = NONE;
   StopObservingIfDone();
 }

content/browser/web_contents/aura/overscroll_navigation_overlay.h

@@ -58,6 +58,8 @@ class CONTENT_EXPORT OverscrollNavigationOverlay
   FRIEND_TEST_ALL_PREFIXES(OverscrollNavigationOverlayTest, OverlayWindowSwap);
   FRIEND_TEST_ALL_PREFIXES(OverscrollNavigationOverlayTest,
                            CloseDuringAnimation);
+  FRIEND_TEST_ALL_PREFIXES(OverscrollNavigationOverlayTest,
+                           ImmediateLoadOnNavigate);
 
   // Resets state and starts observing |web_contents_| for page load/paint
   // updates. This function makes sure that the screenshot window is stacked

content/browser/web_contents/aura/overscroll_navigation_overlay_unittest.cc

@@ -31,6 +31,29 @@
 
 namespace content {
 
+// Forces web contents to complete web page load as soon as navigation starts.
+class ImmediateLoadObserver : WebContentsObserver {
+ public:
+  explicit ImmediateLoadObserver(TestWebContents* contents)
+      : contents_(contents) {
+    Observe(contents);
+  }
+  ~ImmediateLoadObserver() override {}
+
+  void DidStartNavigationToPendingEntry(
+      const GURL& url,
+      NavigationController::ReloadType reload_type) override {
+    // Simulate immediate web page load.
+    contents_->TestSetIsLoading(false);
+    Observe(nullptr);
+  }
+
+ private:
+  TestWebContents* contents_;
+
+  DISALLOW_COPY_AND_ASSIGN(ImmediateLoadObserver);
+};
+
 // A subclass of TestWebContents that offers a fake content window.
 class OverscrollTestWebContents : public TestWebContents {
  public:
@@ -331,6 +354,18 @@ TEST_F(OverscrollNavigationOverlayTest, CloseDuringAnimation) {
   // Ensure a clean close.
 }
 
+// Tests that we can handle the case when the load completes as soon as the
+// navigation is started.
+TEST_F(OverscrollNavigationOverlayTest, ImmediateLoadOnNavigate) {
+  PerformBackNavigationViaSliderCallbacks();
+  // This observer will force the page load to complete as soon as the
+  // navigation starts.
+  ImmediateLoadObserver immediate_nav(contents());
+  GetOverlay()->owa_->OnOverscrollModeChange(OVERSCROLL_NONE, OVERSCROLL_EAST);
+  // This will start and immediately complete the navigation.
+  GetOverlay()->owa_->OnOverscrollComplete(OVERSCROLL_EAST);
+  EXPECT_FALSE(GetOverlay()->window_.get());
+}
 
 // Tests that swapping the overlay window at the end of a gesture caused by the
 // start of a new overscroll does not crash and the events still reach the new