(not for commit) Simplified multi-threaded Ganesh with lock on Flush only.

(not for commit) Simplified multi-threaded Ganesh with lock on Flush only.

--enable-threaded-gpu-rasterization to turn on (default).
--disable-threaded-gpu-rasterization to turn off.

This must be applied on top of https://codereview.chromium.org/889063002/ "Create 2nd ContextProvider for Ganesh rasterization."

BUG=

[vmiura, 2015-01-31 00:29:07]

vmiura@chromium.org changed reviewers:
+ hendrikw@chromium.org, vmpstr@chromium.org

[vmiura, 2015-01-31 00:42:38]

This is not commit ready but I'd appreciate it if you can take a look.

The changes are actually pretty simple (once we have the two contexts set up,
which I split into the other patch).

In gpu_tile_task_worker_pool.cc
RasterBufferImpl::Playback() creates the SkSurface, plays back the picture into
it and releases the SkSurface.

GpuTileTaskWorkerPool::ScheduleTasks() runs ShallowFlushCHROMIUM() on the CC's
context just before scheduling tasks to TGR.  This will make sure the Worker's
context sees any new Resource GL handles.

GpuTileTaskWorkerPool::CheckForCompletedTasks() runs SafeFlush() on the Worker's
context to make sure the CC's context sees updates from any completed tasks.

In terms of settings when in GPU raster with --enable-tile-prepare:

- Same old Ganesh size tiles
- Memory policy of CUTOFF_ALLOW_NICE_TO_HAVE so only NOW + SOON tiles are
rendered.  This bounds how much the worker thread floods the GPU.  It may be
nice if we could switch to CUTOFF_ALLOW_EVERYTHING on a frame that is otherwise
otherwise idle.
- Skewport distance is 0.2 seconds (seems to work well, haven't spent much time
trying other values).