Support single-threaded impl-side painting

cc/resources/rasterizer.h

Index: cc/resources/rasterizer.h
diff --git a/cc/resources/rasterizer.h b/cc/resources/rasterizer.h
index 526058c338d01d42ef7fcb56b587f648c8103537..224ec32409c317be3100e8c91918388e20b06312 100644
--- a/cc/resources/rasterizer.h
+++ b/cc/resources/rasterizer.h
@@ -152,6 +152,9 @@ class CC_EXPORT Rasterizer {
   // 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;