Don't update toolbar alpha when omnibox is focused

The toolbar's buttons are updated on screen rotation. In particular
their alpha value is set to 1 even if the omnibox is focused. It
displays the back/forward button as they are hidden during the animation
of the omnibox focus.
This CL fixes it by not updating the alpha value if the omnibox is
focused.

Bug: 788816
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I604126cb2f80095f35229c0a3c2ae1a14deb3225
Reviewed-on: https://chromium-review.googlesource.com/796071
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520607}

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

@@ -1339,7 +1339,7 @@ using ios::material::TimingFunction;
 
 - (void)updateToolbarAlphaForFrame:(CGRect)frame {
   // Don't update the toolbar buttons if we are animating for a transition.
-  if (self.view.animatingTransition)
+  if (self.view.animatingTransition || [self isOmniboxFirstResponder])
     return;
 
   CGFloat distanceOffscreen =