Enable partial texture upload support for browser compositor with Surfaces

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 fe85134336c21be3891e6fa31f9b5cb0285e7c4f..66c0bb0534bc239259d5a29697e0388a138859e2 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -290,6 +290,8 @@ void LayerTreeHostImpl::BeginMainFrameAborted(CommitEarlyOutReason reason) {
 
 void LayerTreeHostImpl::BeginCommit() {
   TRACE_EVENT0("cc", "LayerTreeHostImpl::BeginCommit");
+  DCHECK(output_surface_);

[danakj, 2015/01/08 21:04:34]

nit: no need to dcheck, we'll crash if it's null on the next line.

why is this at begin commit instead of commitcomplete? (you want this to be
before the pending tree starts to update things right?)

+  output_surface_->ForceReclaimResources();
 
   if (UsePendingTreeForSync())
     CreatePendingTree();