Make sure all animations have ended before switching layouts

It is possible to make chrome switch layouts mid-animation. This
patch kills running animations in LayoutManager#startShowing to avoid
starting new animations in a bad state.

BUG=818017,820899

Change-Id: Ie591beedbf855a2ffd31baf849a334dd5230742b
Reviewed-on: https://chromium-review.googlesource.com/958333Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543135}

chrome/android/java/src/org/chromium/chrome/browser/compositor/layouts/LayoutManager.java

@@ -808,6 +808,7 @@ public class LayoutManager implements LayoutUpdateHost, LayoutProvider,
         Layout oldLayout = getActiveLayout();
         if (oldLayout != layout) {
             if (oldLayout != null) {
+                oldLayout.forceAnimationToFinish();
                 oldLayout.detachViews();
             }
             layout.contextChanged(mHost.getContext());