Fix toolbar hiding while the mouse is still over it when the system uses a dark menu bar.

FullscreenToolbarMouseTracker has a method, -updateToolbarFrame:, which
sets trackingAreaFrame_ based on contentView_'s bounds, then calls
-updateTrackingArea. However, -updateTrackingArea is responsible for
setting contentView_, so its value is stale (nil, in this case) when
-updateToolbarFrame: looks at it.

trackingAreaFrame_ only becomes correct if -updateToolbarFrame: is
called a second time. I haven't discovered why, but it looks like we get
extra events from the light menu bar, but not the dark one, which masked
the bug.

Fix the bug by having -updateToolbarFrame: find the content view itself
instead of relying on the contentView_ ivar (which seems to mainly be
used to remember which view currently has a tracking area).

Bug: 749224
Change-Id: I3c923d09590e3416ebcf8bbb428d1dd44f62fab7
Reviewed-on: https://chromium-review.googlesource.com/627296
Commit-Queue: Sidney San Martín <sdy@chromium.org>
Reviewed-by: Sarah Chan <spqchan@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496574}

chrome/browser/ui/cocoa/fullscreen/fullscreen_toolbar_mouse_tracker.mm

@@ -88,7 +88,8 @@ initWithFullscreenToolbarController:(FullscreenToolbarController*)owner
 }
 
 - (void)updateToolbarFrame:(NSRect)frame {
-  NSRect contentBounds = [contentView_ bounds];
+  NSRect contentBounds =
+      [[[[owner_ browserWindowController] window] contentView] bounds];
   trackingAreaFrame_ = frame;
   trackingAreaFrame_.origin.y -= kTrackingAreaAdditionalThreshold;
   trackingAreaFrame_.size.height =