cc: Fix WeakPtrFactory::GetWeakPtr() race when using multiple raster worker threads.

This prevents multiple threads from being able to call GetWeakPtr()
at the same time by making sure the lock is acquired when this is
called.

R=vmpstr@chromium.org
BUG=

Review URL: https://codereview.chromium.org/700843004

Cr-Commit-Position: refs/heads/master@{#302717}

cc/resources/one_copy_raster_worker_pool.cc

@@ -275,9 +275,6 @@ OneCopyRasterWorkerPool::PlaybackAndScheduleCopyOnWorkerThread(
     const RasterSource* raster_source,
     const gfx::Rect& rect,
     float scale) {
-  CopySequenceNumber sequence;
-
-  {
   base::AutoLock lock(lock_);
 
   int failed_attempts = 0;
@@ -314,22 +311,24 @@ OneCopyRasterWorkerPool::PlaybackAndScheduleCopyOnWorkerThread(
   {
     base::AutoUnlock unlock(lock_);
 
-      gfx::GpuMemoryBuffer* gpu_memory_buffer =
-          write_lock->GetGpuMemoryBuffer();
+    gfx::GpuMemoryBuffer* gpu_memory_buffer = write_lock->GetGpuMemoryBuffer();
     if (gpu_memory_buffer) {
-        RasterWorkerPool::PlaybackToMemory(
-            gpu_memory_buffer->Map(), src->format(), src->size(),
-            gpu_memory_buffer->GetStride(), raster_source, rect, scale);
+      RasterWorkerPool::PlaybackToMemory(gpu_memory_buffer->Map(),
+                                         src->format(),
+                                         src->size(),
+                                         gpu_memory_buffer->GetStride(),
+                                         raster_source,
+                                         rect,
+                                         scale);
       gpu_memory_buffer->Unmap();
     }
   }
 
-    // Acquire a sequence number for this copy operation.
-    sequence = next_copy_operation_sequence_++;
-
   pending_copy_operations_.push_back(
       make_scoped_ptr(new CopyOperation(write_lock.Pass(), src.Pass(), dst)));
-  }
+
+  // Acquire a sequence number for this copy operation.
+  CopySequenceNumber sequence = next_copy_operation_sequence_++;
 
   // Post task that will advance last flushed copy operation to |sequence|
   // if we have reached the flush period.