Update the toolbar alpha only on frame changes

When the frame of the toolbar changes, the alpha of the buttons is
updated. But the new alpha value is computed with only the y value
of the frame.

So instead of comparing the whole frame to see if it changed, this
CL only compares the y value of the origins.

Bug: 793011
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ie10d2f6d4639edb040999e71794d0b30347cf4be
Reviewed-on: https://chromium-review.googlesource.com/817285
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523112}

ios/chrome/browser/ui/toolbar/web_toolbar_controller.mm

@@ -133,8 +133,8 @@ using ios::material::TimingFunction;
   // If set to |YES|, disables animations that tests would otherwise trigger.
   BOOL _unitTesting;
 
-  // Keeps track of the last known toolbar frame.
-  CGRect _lastKnownToolbarFrame;
+  // Keeps track of the last known toolbar Y origin.
+  CGFloat _lastKnownToolbarYOrigin;
 
   // Keeps track of last known trait collection used by the subviews.
   UITraitCollection* _lastKnownTraitCollection;
@@ -999,10 +999,10 @@ using ios::material::TimingFunction;
 
 - (void)toolbarDidLayout {
   CGRect frame = self.view.frame;
-  if (CGRectEqualToRect(_lastKnownToolbarFrame, frame))
+  if (frame.origin.y == _lastKnownToolbarYOrigin)
     return;
   [self updateToolbarAlphaForFrame:frame];
-  _lastKnownToolbarFrame = frame;
+  _lastKnownToolbarYOrigin = frame.origin.y;
 }
 
 - (void)windowDidChange {