cc: Allow the main thread to cancel commits

cc/layers/layer.cc

Index: cc/layers/layer.cc
diff --git a/cc/layers/layer.cc b/cc/layers/layer.cc
index ed979a9a41301c26e18e9373a25bb9bf1e02a269..694813982b01c8759604e089d953920f1e2dfc87 100644
--- a/cc/layers/layer.cc
+++ b/cc/layers/layer.cc
@@ -115,6 +115,11 @@ void Layer::SetLayerTreeHost(LayerTreeHost* host) {
     layer_tree_host_->set_needs_filter_context();
 }
 
+void Layer::SetNeedsUpdate() {
+  if (layer_tree_host_)
+    layer_tree_host_->SetNeedsUpdateLayers();
+}
+
 void Layer::SetNeedsCommit() {
   if (ignore_set_needs_commit_)
     return;
@@ -517,6 +522,8 @@ void Layer::SetScrollOffset(gfx::Vector2d scroll_offset) {
 
 void Layer::SetScrollOffsetFromImplSide(gfx::Vector2d scroll_offset) {
   DCHECK(IsPropertyChangeAllowed());
+  // This function only gets called during a begin frame, so there
+  // is no need to call SetNeedsUpdate here.
   DCHECK(layer_tree_host_ && layer_tree_host_->CommitRequested());
   if (scroll_offset_ == scroll_offset)
     return;
@@ -620,11 +627,8 @@ void Layer::SetNeedsDisplayRect(const gfx::RectF& dirty_rect) {
   update_rect_.Union(dirty_rect);
   needs_display_ = true;
 
-  // Simply mark the contents as dirty. For non-root layers, the call to
-  // SetNeedsCommit will schedule a fresh compositing pass.
-  // For the root layer, SetNeedsCommit has no effect.
   if (DrawsContent() && !update_rect_.IsEmpty())
-    SetNeedsCommit();
+    SetNeedsUpdate();
 }
 
 bool Layer::DescendantIsFixedToContainerLayer() const {