Allow task pools to reason about transparency.

cc/raster/zero_copy_tile_task_worker_pool.cc

Index: cc/raster/zero_copy_tile_task_worker_pool.cc
diff --git a/cc/raster/zero_copy_tile_task_worker_pool.cc b/cc/raster/zero_copy_tile_task_worker_pool.cc
index 2e3dca1407f3358802bbd612554bf5f22ea46c4d..0a6e80e7d1c0880f7476c5a3b5f384a6f4981b8c 100644
--- a/cc/raster/zero_copy_tile_task_worker_pool.cc
+++ b/cc/raster/zero_copy_tile_task_worker_pool.cc
@@ -65,21 +65,24 @@ class RasterBufferImpl : public RasterBuffer {
 scoped_ptr<TileTaskWorkerPool> ZeroCopyTileTaskWorkerPool::Create(
     base::SequencedTaskRunner* task_runner,
     TaskGraphRunner* task_graph_runner,
-    ResourceProvider* resource_provider) {
+    ResourceProvider* resource_provider,
+    bool use_rgba_4444_texture_format) {
   return make_scoped_ptr<TileTaskWorkerPool>(new ZeroCopyTileTaskWorkerPool(
-      task_runner, task_graph_runner, resource_provider));
+      task_runner, task_graph_runner, resource_provider,
+      use_rgba_4444_texture_format));
 }
 
 ZeroCopyTileTaskWorkerPool::ZeroCopyTileTaskWorkerPool(
     base::SequencedTaskRunner* task_runner,
     TaskGraphRunner* task_graph_runner,
-    ResourceProvider* resource_provider)
+    ResourceProvider* resource_provider,
+    bool use_rgba_4444_texture_format)
     : task_runner_(task_runner),
       task_graph_runner_(task_graph_runner),
       namespace_token_(task_graph_runner->GetNamespaceToken()),
       resource_provider_(resource_provider),
-      task_set_finished_weak_ptr_factory_(this) {
-}
+      use_rgba_4444_texture_format_(use_rgba_4444_texture_format),
+      task_set_finished_weak_ptr_factory_(this) {}
 
 ZeroCopyTileTaskWorkerPool::~ZeroCopyTileTaskWorkerPool() {
 }
@@ -179,12 +182,17 @@ void ZeroCopyTileTaskWorkerPool::CheckForCompletedTasks() {
   completed_tasks_.clear();
 }
 
-ResourceFormat ZeroCopyTileTaskWorkerPool::GetResourceFormat() const {
-  return resource_provider_->memory_efficient_texture_format();
+ResourceFormat ZeroCopyTileTaskWorkerPool::GetResourceFormat(
+    bool must_support_alpha) const {
+  return use_rgba_4444_texture_format_
+             ? RGBA_4444
+             : resource_provider_->best_texture_format();
 }
 
-bool ZeroCopyTileTaskWorkerPool::GetResourceRequiresSwizzle() const {
-  return !PlatformColor::SameComponentOrder(GetResourceFormat());
+bool ZeroCopyTileTaskWorkerPool::GetResourceRequiresSwizzle(
+    bool must_support_alpha) const {
+  return !PlatformColor::SameComponentOrder(
+      GetResourceFormat(must_support_alpha));
 }
 
 scoped_ptr<RasterBuffer> ZeroCopyTileTaskWorkerPool::AcquireBufferForRaster(