HACK: Rasterize tiles immediately if there are only a few of them

cc/tile_manager.cc

Index: cc/tile_manager.cc
diff --git a/cc/tile_manager.cc b/cc/tile_manager.cc
index 612e39dcb3c87cbc9782a9176d6552e3ad78678f..b87825feb2a0fa5c393debe7bf18afe4587e46fb 100644
--- a/cc/tile_manager.cc
+++ b/cc/tile_manager.cc
@@ -534,7 +534,7 @@ void TileManager::DispatchMoreTasks() {
     if (managed_state.pending_pixel_refs.empty()) {
       if (!CanDispatchRasterTask(*it))
         return;
-      DispatchOneRasterTask(*it);
+      DispatchOneRasterTask(*it, false);
       tiles_with_image_decoding_tasks_.erase(it++);
     } else {
       ++it;
@@ -552,7 +552,9 @@ void TileManager::DispatchMoreTasks() {
     } else {
       if (!CanDispatchRasterTask(tile))
         return;
-      DispatchOneRasterTask(tile);
+      bool immediate =
+          tiles_that_need_to_be_rasterized_.size() < 5;
+      DispatchOneRasterTask(tile, immediate);
     }
     tiles_that_need_to_be_rasterized_.pop_back();
   }
@@ -635,7 +637,8 @@ void TileManager::OnImageDecodeTaskCompleted(
   DispatchMoreTasks();
 }
 
-void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile) {
+void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile,
+                                        bool immediate) {
   TRACE_EVENT0("cc", "TileManager::DispatchOneRasterTask");
   ManagedTileState& managed_tile_state = tile->managed_state();
   DCHECK(managed_tile_state.can_use_gpu_memory);
@@ -650,6 +653,19 @@ void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile) {
 
   ResourceProvider::ResourceId resource_id = resource->id();
 
+  if (immediate) {
+    RunRasterTask(resource_pool_->resource_provider()->mapPixelBuffer(
+                      resource_id),
+                  tile->content_rect_,
+                  tile->contents_scale(),
+                  tile->picture_pile(),
+                  &rendering_stats_);
+    TileManager::OnRasterTaskCompleted(tile, resource.Pass(),
+                                       manage_tiles_call_count_,
+                                       false);
+    return;
+  }
+
   raster_worker_pool_->PostRasterTaskAndReply(
       tile->picture_pile(),
       base::Bind(&TileManager::RunRasterTask,
@@ -661,13 +677,15 @@ void TileManager::DispatchOneRasterTask(scoped_refptr<Tile> tile) {
                  base::Unretained(this),
                  tile,
                  base::Passed(&resource),
-                 manage_tiles_call_count_));
+                 manage_tiles_call_count_,
+                 true));
 }
 
 void TileManager::OnRasterTaskCompleted(
     scoped_refptr<Tile> tile,
     scoped_ptr<ResourcePool::Resource> resource,
-    int manage_tiles_call_count_when_dispatched) {
+    int manage_tiles_call_count_when_dispatched,
+    bool dispatch_more) {
   TRACE_EVENT0("cc", "TileManager::OnRasterTaskCompleted");
 
   // Release raster resources.
@@ -710,7 +728,8 @@ void TileManager::OnRasterTaskCompleted(
     DidTileRasterStateChange(tile, IDLE_STATE);
   }
 
-  DispatchMoreTasks();
+  if (dispatch_more)
+    DispatchMoreTasks();
 }
 
 void TileManager::DidFinishTileInitialization(Tile* tile) {