cc: Allow the main thread to cancel commits

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index cdb5f61a82c3866d818fa4cf8ddcefbb6fe3e788..0c82db87ea031a334ea50fe7689cf7d712f7e11d 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -469,6 +469,8 @@ void LayerTreeHost::SetNeedsAnimate() {
   proxy_->SetNeedsAnimate();
 }
 
+void LayerTreeHost::SetNeedsUpdateLayers() { proxy_->SetNeedsUpdateLayers(); }
+
 void LayerTreeHost::SetNeedsCommit() {
   if (!prepaint_callback_.IsCancelled()) {
     TRACE_EVENT_INSTANT0("cc",
@@ -581,6 +583,11 @@ void LayerTreeHost::SetOverdrawBottomHeight(float overdraw_bottom_height) {
   SetNeedsCommit();
 }
 
+void LayerTreeHost::ApplyPageScaleDeltaFromImplSide(float page_scale_delta) {
+  DCHECK(CommitRequested());
+  page_scale_factor_ *= page_scale_delta;
+}
+
 void LayerTreeHost::SetPageScaleFactorAndLimits(float page_scale_factor,
                                                 float min_page_scale_factor,
                                                 float max_page_scale_factor) {
@@ -965,8 +972,8 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
   if (!root_layer_.get())
     return;
 
-  Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
   gfx::Vector2d root_scroll_delta;
+  Layer* root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
 
   for (size_t i = 0; i < info.scrolls.size(); ++i) {
     Layer* layer =
@@ -981,8 +988,22 @@ void LayerTreeHost::ApplyScrollAndScale(const ScrollAndScaleSet& info) {
                                          info.scrolls[i].scroll_delta);
     }
   }
-  if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f)
+
+  if (!root_scroll_delta.IsZero() || info.page_scale_delta != 1.f) {
+    // SetScrollOffsetFromImplSide above could have destroyed the tree,
+    // so re-get this layer before doing anything to it.
+    root_scroll_layer = FindFirstScrollableLayer(root_layer_.get());
+
+    // Preemptively apply the scroll offset and scale delta here before sending
+    // it to the client.  If the client comes back and sets it to the same
+    // value, then the layer can early out without needing a full commit.
+    if (root_scroll_layer) {
+      root_scroll_layer->SetScrollOffsetFromImplSide(
+          root_scroll_layer->scroll_offset() + root_scroll_delta);
+    }
+    ApplyPageScaleDeltaFromImplSide(info.page_scale_delta);
     client_->ApplyScrollAndScale(root_scroll_delta, info.page_scale_delta);
+  }
 }
 
 void LayerTreeHost::StartRateLimiter(WebKit::WebGraphicsContext3D* context3d) {