cc: Do bitmap conversion for RasterBuffer in the worker thread.

cc/resources/rasterizer.cc

Index: cc/resources/rasterizer.cc
diff --git a/cc/resources/rasterizer.cc b/cc/resources/rasterizer.cc
index df7e27ecebcc147cb745db354527c4804d794934..17e07d0193005b914f6b6398877359750e0d91f1 100644
--- a/cc/resources/rasterizer.cc
+++ b/cc/resources/rasterizer.cc
@@ -48,7 +48,7 @@ ImageDecodeTask* ImageDecodeTask::AsImageDecodeTask() { return this; }
 
 RasterTask::RasterTask(const Resource* resource,
                        ImageDecodeTask::Vector* dependencies)
-    : resource_(resource) {
+    : resource_(resource), content_has_changed_(false) {
   dependencies_.swap(*dependencies);
 }
 
@@ -56,6 +56,10 @@ RasterTask::~RasterTask() {}
 
 RasterTask* RasterTask::AsRasterTask() { return this; }
 
+void RasterTask::set_content_has_changed(bool content_has_changed) {
+  content_has_changed_ = content_has_changed;
+}
+
 RasterTaskQueue::Item::Item(RasterTask* task, bool required_for_activation)
     : task(task), required_for_activation(required_for_activation) {}