Plumb an overscroll callback form the compositor to the Android UI thread.

cc/layer_tree_host_impl_unittest.cc

Index: cc/layer_tree_host_impl_unittest.cc
diff --git a/cc/layer_tree_host_impl_unittest.cc b/cc/layer_tree_host_impl_unittest.cc
index d707b46f90622c4c4f88256542e232c218e1e386..2b0f74ac549ea496e79856aea0411c2ed0b9b54f 100644
--- a/cc/layer_tree_host_impl_unittest.cc
+++ b/cc/layer_tree_host_impl_unittest.cc
@@ -911,7 +911,16 @@ TEST_P(LayerTreeHostImplTest, compositorFrameMetadata)
     {
         CompositorFrameMetadata metadata = m_hostImpl->makeCompositorFrameMetadata();
         EXPECT_EQ(gfx::Vector2dF(0.0f, 10.0f), metadata.root_scroll_offset);
+        EXPECT_EQ(gfx::Vector2dF(0.0f, 0.0f), metadata.unused_scroll_delta);
     }
+    // Overscroll should update metadata immediately.
+    m_hostImpl->scrollBy(gfx::Point(), gfx::Vector2d(0, 50));
+    {
+        CompositorFrameMetadata metadata = m_hostImpl->makeCompositorFrameMetadata();
+        EXPECT_EQ(gfx::Vector2dF(0.0f, 50.0f), metadata.root_scroll_offset);
+        EXPECT_EQ(gfx::Vector2dF(0.0f, 10.0f), metadata.unused_scroll_delta);
+    }
+    m_hostImpl->scrollBy(gfx::Point(), gfx::Vector2d(0, -40));
     m_hostImpl->scrollEnd();
 
     {