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 8a3e644bb1109fe7348501cde4fee91fd810f34e..0177c730d2a5dd1aca1e48ead26062fed5db1d46 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -21,6 +21,7 @@
 #include "cc/animation/layer_animation_controller.h"
 #include "cc/base/math_util.h"
 #include "cc/debug/devtools_instrumentation.h"
+#include "cc/debug/frame_viewer_instrumentation.h"
 #include "cc/debug/rendering_stats_instrumentation.h"
 #include "cc/input/layer_selection_bound.h"
 #include "cc/input/page_scale_animation.h"
@@ -268,6 +269,14 @@ void LayerTreeHost::FinishCommitOnImplThread(LayerTreeHostImpl* host_impl) {
     contents_texture_manager_->ReduceMemory(host_impl->resource_provider());
   }
 
+  bool is_new_trace;
+  TRACE_EVENT_IS_NEW_TRACE(&is_new_trace);
+  if (is_new_trace &&

[danakj, 2015/03/17 17:08:53]

I think you need if (root_layer()) as well here.

[caseq, 2015/03/17 17:44:55]

Thanks, done!
FWIW we seem to unconditionally arrive to a call to
TreeSynchronizer::PushProperties() at the end which does a DCHECK() on its
source root layer (sure, there's still pretty important difference in case of
release :))

[danakj, 2015/03/17 18:36:05]

I think it does an early out:
https://code.google.com/p/chromium/codesearch#chromium/src/cc/trees/tree_sync...

We can definitely commit without a root layer.

+      frame_viewer_instrumentation::IsTracingLayerTreeSnapshots()) {
+    LayerTreeHostCommon::CallFunctionForSubtree(
+        root_layer(), [](Layer* layer) { layer->DidBeginTracing(); });
+  }
+
   LayerTreeImpl* sync_tree = host_impl->sync_tree();
 
   if (next_commit_forces_redraw_) {