Support single-threaded impl-side painting

cc/resources/rasterizer.h

 // Copyright 2014 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.
 
 #ifndef CC_RESOURCES_RASTERIZER_H_
 #define CC_RESOURCES_RASTERIZER_H_
 
 #include <bitset>
 #include <vector>
 
@@ skipping 134 matching lines @@
   // CheckForCompletedTasks() is called.
   virtual void Shutdown() = 0;
 
   // Schedule running of raster tasks in |queue| and all dependencies.
   // Previously scheduled tasks that are not in |queue| will be canceled unless
   // already running. Once scheduled, reply callbacks are guaranteed to run for
   // all tasks even if they later get canceled by another call to
   // ScheduleTasks().
   virtual void ScheduleTasks(RasterTaskQueue* queue) = 0;
 
+  // Blocks until all currently scheduled tasks have completed.
+  virtual void WaitForTasksToFinishRunning() = 0;

[reveman, 2014/10/27 19:15:49]

I don't think we should add this to the Rasterizer interface when it can only be
supported by the 0-copy implementation.

Here's an alternative solution:

1. In single-threaded impl-side painting mode, have LTHI create a
ZeroCopyRasterWorkerPool instance using a non-threaded TaskGraphRunner instance.

2. In SingleThreadProxy::CompositeImmediately, after calling
LayerTreeHostImpl::ManageTiles(), call LayerTreeHostImpl::InitializeAllTiles(),
which in turn will call TaskGraphRunner::RunUntilIdle() on the task graph runner
instance given to the ZeroCopyRasterWorkerPool instance.

This should have the same effect but would remove the need for this plumbing and
having incorrect implementations of this interface in all but one Rasterizer
implementation.

This would run all raster on the compositor thread instead of worker threads. If
it needs to run on worker threads then adding a WaitForTasksToFinishRunning to
only the ZeroCopyRasterWorkerPool class is an option.

[enne (OOO), 2014/10/27 20:23:24]

What do you mean by non-threaded? Is this something I would add, or is there
some way to specify this that I can't see?
What do you mean by LayerTreeHostImpl::InitializeAllTiles?  Is this some new
function that I would write or are you referring to something?

+
   // Check for completed tasks and dispatch reply callbacks.
   virtual void CheckForCompletedTasks() = 0;
 
  protected:
   virtual ~Rasterizer() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RASTERIZER_H_