CC: Force push properties for all layers when tracing is started

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 abc63fc932eebfd8297d1bd0e020e9c3040b72a1..1e8fb7a833a9ee02b32c41afa80a93f42dfc4b8e 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -215,6 +215,10 @@ static void LayerTreeHostOnOutputSurfaceCreatedCallback(Layer* layer) {
   layer->OnOutputSurfaceCreated();
 }
 
+static void LayerTreeHostDidBeginTracingCallback(Layer* layer) {
+  layer->DidBeginTracing();
+}
+
 void LayerTreeHost::DeleteContentsTexturesOnImplThread(
     ResourceProvider* resource_provider) {
   DCHECK(proxy_->IsImplThread());
@@ -298,6 +302,23 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
 
   sync_tree->set_source_frame_number(source_frame_number());
 
+  bool is_new_trace;
+  TRACE_EVENT_IS_NEW_TRACE(&is_new_trace);
+  if (is_new_trace) {
+    // We're about to start tracing snaphsots of layer tree, so force
+    // push properties for all layers to propagate LayerDebugInfo to

[danakj, 2015/03/06 18:50:12]

I see, LayerDebugInfo is a type, up in ui/compositor/ but that only is relevant
for the ui compositor, not for the renderer, so referring to it here is a
layering violation. Can you make this comment more general so it's accurate for
the renderer too?

+    // the impl side.
+    bool will_snapshot_layer_tree;
+    TRACE_EVENT_CATEGORY_GROUP_ENABLED(
+        TRACE_DISABLED_BY_DEFAULT("cc.debug") "," TRACE_DISABLED_BY_DEFAULT(

[danakj, 2015/03/06 18:50:12]

I don't like that these magic categories appear here and just kinda "know" what
will get traced later. How can we ensure this isn't going to get out of sync at
some point and cause bugs?

(I don't even see the devtools.timeline.layers mentioned here:
https://code.google.com/p/chromium/codesearch#chromium/src/cc/layers/layer_im...)

+            "devtools.timeline.layers"),
+        &will_snapshot_layer_tree);
+    if (will_snapshot_layer_tree) {
+      LayerTreeHostCommon::CallFunctionForSubtree(
+          root_layer(), base::Bind(LayerTreeHostDidBeginTracingCallback));

[danakj, 2015/03/06 18:50:12]

This will need a rebase.

+    }
+  }
+
   if (needs_full_tree_sync_) {
     sync_tree->SetRootLayer(TreeSynchronizer::SynchronizeTrees(
         root_layer(), sync_tree->DetachLayerTree(), sync_tree));