Add an "isInsideVSync" check.

ui/android/java/src/org/chromium/ui/VSyncMonitor.java

Index: ui/android/java/src/org/chromium/ui/VSyncMonitor.java
diff --git a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
index 7d9d8a6ac98e53d8ddda213c4bba96a070426e76..af8f87b5ffd898c832a1fa2f0db7cd590e50ca89 100644
--- a/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
+++ b/ui/android/java/src/org/chromium/ui/VSyncMonitor.java
@@ -24,6 +24,8 @@ public class VSyncMonitor {
     private static final long NANOSECONDS_PER_MILLISECOND = 1000000;
     private static final long NANOSECONDS_PER_MICROSECOND = 1000;
 
+    private boolean mInsideVSync = false;
+
     /**
      * VSync listener class
      */
@@ -150,16 +152,32 @@ public class VSyncMonitor {
         mGoodStartingPointNano = goodStartingPointNano;
     }
 
+    /**
+     * @return true if onVSync handler is executing. If onVSync handler
+     * introduces invalidations, View#invalidate() should be called. If
+     * View#postInvalidateOnAnimation is called instead, the corresponding onDraw
+     * will be delayed by one frame. The embedder of VSyncMonitor should check
+     * this value if it wants to post an invalidation.
+     */
+    public boolean isInsideVSync() {
+        return mInsideVSync;
+    }
+
     private long getCurrentNanoTime() {
         return System.nanoTime();
     }
 
     private void onVSyncCallback(long frameTimeNanos, long currentTimeNanos) {
         assert mHaveRequestInFlight;
+        mInsideVSync = true;
         mHaveRequestInFlight = false;
         mLastVSyncCpuTimeNano = currentTimeNanos;
-        if (mListener != null) {
-            mListener.onVSync(this, frameTimeNanos / NANOSECONDS_PER_MICROSECOND);
+        try {
+            if (mListener != null) {
+                mListener.onVSync(this, frameTimeNanos / NANOSECONDS_PER_MICROSECOND);
+            }
+        } finally {
+            mInsideVSync = false;
         }
     }