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 ff4d0079fe3983e8096afe381a0cafaa7035ad7f..68fdf792f45b566764c7d804cf48b1de46b24b84 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -465,6 +465,10 @@ void LayerTreeHost::SetNeedsAnimate() {
   proxy_->SetNeedsAnimate();
 }
 
+void LayerTreeHost::SetNeedsUpdateLayers() {
+  proxy_->SetNeedsUpdateLayers();
+}
+
 void LayerTreeHost::SetNeedsCommit() {
   if (!prepaint_callback_.IsCancelled()) {
     TRACE_EVENT_INSTANT0("cc",
@@ -580,15 +584,21 @@ void LayerTreeHost::SetOverdrawBottomHeight(float overdraw_bottom_height) {
 void LayerTreeHost::SetPageScaleFactorAndLimits(float page_scale_factor,
                                                 float min_page_scale_factor,
                                                 float max_page_scale_factor) {
-  if (page_scale_factor == page_scale_factor_ &&
+  bool same_page_scale_factor = page_scale_factor == page_scale_factor_;
+  bool same_page_scale_limits =
       min_page_scale_factor == min_page_scale_factor_ &&
-      max_page_scale_factor == max_page_scale_factor_)
+      max_page_scale_factor == max_page_scale_factor_;
+  if (same_page_scale_factor && same_page_scale_limits)
     return;
 
   page_scale_factor_ = page_scale_factor;
   min_page_scale_factor_ = min_page_scale_factor;
   max_page_scale_factor_ = max_page_scale_factor;
-  SetNeedsCommit();
+
+  if (same_page_scale_limits)
+    SetNeedsUpdateLayers();
+  else
+    SetNeedsCommit();
 }
 
 void LayerTreeHost::SetVisible(bool visible) {
@@ -948,8 +958,8 @@ 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 =
@@ -964,8 +974,21 @@ 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 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);
+    }
     client_->ApplyScrollAndScale(root_scroll_delta, info.page_scale_delta);
+  }
 }
 
 void LayerTreeHost::StartRateLimiter(WebKit::WebGraphicsContext3D* context3d) {