When possible, draw the toolbar shadow with the compositor

chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java

Index: chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java b/chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java
index 49d94a65db23ae1f957b81169c0e7a1cd290ba69..c3eed60d43fe3a069cb7f7f1a9b752b9b525a76c 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/compositor/CompositorViewHolder.java
@@ -115,6 +115,7 @@ public class CompositorViewHolder extends CoordinatorLayout
     private View mView;
 
     private TabObserver mTabObserver;
+    private FrameObserver mFrameObserver;
     private boolean mEnableCompositorTabStrip;
 
     // Cache objects that should not be created frequently.
@@ -126,6 +127,16 @@ public class CompositorViewHolder extends CoordinatorLayout
     private boolean mHasDrawnOnce = false;
 
     /**
+     * An interface for outside classes to be notified when a frame from the compositor is ready.
+     */
+    public interface FrameObserver {
+        /**
+         * Notification of the compositor having drawn a frame.
+         */
+        public void onCompositorFrame();
+    }
+
+    /**
      * This view is created on demand to display debugging information.
      */
     private static class DebugOverlay extends View {
@@ -231,6 +242,13 @@ public class CompositorViewHolder extends CoordinatorLayout
     }
 
     /**
+     * @param observer The object observing frame drawing.
+     */
+    public void setFrameObserver(FrameObserver observer) {
+        mFrameObserver = observer;
+    }
+
+    /**
      * @param layoutManager The {@link LayoutManager} instance that will be driving what
      *                      shows in this {@link CompositorViewHolder}.
      */
@@ -575,6 +593,7 @@ public class CompositorViewHolder extends CoordinatorLayout
         }
 
         mHasDrawnOnce = true;
+        if (mFrameObserver != null) mFrameObserver.onCompositorFrame();
 
         mPendingSwapBuffersCount = pendingSwapBuffersCount;