Fix GetSnapshotFromRenderer stalling when capturing while between navigating to certain pages.

cc/trees/thread_proxy.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/thread_proxy.h"
 
 #include <string>
 
 #include "base/auto_reset.h"
 #include "base/bind.h"
@@ skipping 94 matching lines @@
 ThreadProxy::~ThreadProxy() {
   TRACE_EVENT0("cc", "ThreadProxy::~ThreadProxy");
   DCHECK(IsMainThread());
   DCHECK(!started_);
 }
 
 bool ThreadProxy::CompositeAndReadback(void* pixels, gfx::Rect rect) {
   TRACE_EVENT0("cc", "ThreadProxy::CompositeAndReadback");
   DCHECK(IsMainThread());
   DCHECK(layer_tree_host_);
-  DCHECK(!defer_commits_);
+
+  if (defer_commits_) {
+    TRACE_EVENT0("cc", "CompositeAndReadback_DeferCommit");
+    return false;
+  }
 
   if (!layer_tree_host_->InitializeOutputSurfaceIfNeeded()) {
     TRACE_EVENT0("cc", "CompositeAndReadback_EarlyOut_LR_Uninitialized");
     return false;
   }
 
   // Perform a synchronous commit.
   {
     DebugScopedSetMainThreadBlocked main_thread_blocked(this);
     CompletionEvent begin_frame_sent_to_main_thread_completion;
@@ skipping 1380 matching lines @@
     DCHECK(layer_tree_host_impl_->settings().impl_side_painting);
     completion_event_for_commit_held_on_tree_activation_->Signal();
     completion_event_for_commit_held_on_tree_activation_ = NULL;
   }
 
   commit_to_activate_duration_history_.InsertSample(
       base::TimeTicks::HighResNow() - commit_complete_time_);
 }
 
 }  // namespace cc