compositor-worker: Initialize CW machinery plumbing to compositor and fire CW rAF callbacks.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 17a3e6102ef677d8f9d6c6b16425fc630682d05a..8b5e48702e9cb458a29b2935f7c33fce04e58091 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -462,6 +462,8 @@ void LayerTreeHostImpl::AnimateInternal(bool active_tree) {
   did_animate |= AnimateTopControls(monotonic_time);
 
   if (active_tree) {
+    did_animate |= Mutate(monotonic_time);
+
     // Animating stuff can change the root scroll offset, so inform the
     // synchronous input handler.
     UpdateRootLayerStateForSynchronousInputHandler();
@@ -473,6 +475,20 @@ void LayerTreeHostImpl::AnimateInternal(bool active_tree) {
   }
 }
 
+bool LayerTreeHostImpl::Mutate(base::TimeTicks monotonic_time) {
+  if (!mutator_)
+    return false;
+  TRACE_EVENT0("compositor-worker", "LayerTreeHostImpl::Mutate");
+  if (mutator_->Mutate(monotonic_time))
+    client_->SetNeedsOneBeginImplFrameOnImplThread();
+  return true;
+}
+
+void LayerTreeHostImpl::SetNeedsMutate() {
+  TRACE_EVENT0("compositor-worker", "LayerTreeHostImpl::SetNeedsMutate");
+  client_->SetNeedsOneBeginImplFrameOnImplThread();
+}
+
 bool LayerTreeHostImpl::PrepareTiles() {
   if (!tile_priorities_dirty_)
     return false;
@@ -1990,6 +2006,7 @@ void LayerTreeHostImpl::ActivateSyncTree() {
     // If we commit to the active tree directly, this is already done during
     // commit.
     ActivateAnimations();
+    Mutate(CurrentBeginFrameArgs().frame_time);

[ajuma, 2016/05/05 14:35:45]

Is this done here (rather than in the call to AnimateInternal that happens
during CommitComplete) because CompositorWorker takes active tree values as
input so we need to wait for activation? Please add a comment.

[flackr, 2016/05/05 15:13:14]

Yes, comment added. However, afaik this will be the second time we run Mutate
during this frame because we don't know if the commit / activation will come in
in time for the next frame.

   } else {
     active_tree_->ProcessUIResourceRequestQueue();
   }
@@ -2238,10 +2255,14 @@ void LayerTreeHostImpl::CreateResourceAndRasterBufferProvider(
       settings_.renderer_settings.preferred_tile_format);
 }
 
-void LayerTreeHostImpl::SetLayerTreeMutator(LayerTreeMutator* mutator) {
+void LayerTreeHostImpl::SetLayerTreeMutator(
+    std::unique_ptr<LayerTreeMutator> mutator) {
+  if (mutator == mutator_)
+    return;
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("compositor-worker"),
                "LayerTreeHostImpl::SetLayerTreeMutator");
-  mutator_ = mutator;
+  mutator_ = std::move(mutator);
+  mutator_->SetClient(this);
 }
 
 void LayerTreeHostImpl::CleanUpTileManagerAndUIResources() {
@@ -3101,6 +3122,9 @@ InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     UpdateRootLayerStateForSynchronousInputHandler();
   }
 
+  // Update compositor worker mutations which may respond to scrolling.
+  Mutate(CurrentBeginFrameArgs().frame_time);
+
   return scroll_result;
 }
 
@@ -3848,8 +3872,11 @@ void LayerTreeHostImpl::SetTreeLayerScrollOffsetMutated(
     return;
 
   LayerImpl* layer = tree->LayerById(layer_id);
-  if (layer)
+  if (layer) {
     layer->OnScrollOffsetAnimated(scroll_offset);
+    // Run mutation callbacks to respond to updated scroll offset.
+    Mutate(CurrentBeginFrameArgs().frame_time);
+  }
 }
 
 void LayerTreeHostImpl::TreeLayerTransformIsPotentiallyAnimatingChanged(