Add multithreaded mode to quilt mode in DM.

Add multithreaded mode to quilt mode in DM.

Default off for now.  Something to work toward.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/ec2ae5878d0c4fe3492b6866eaa3b946e526f1a3

[mtklein_C, 2014-06-30 22:28:17]

Crashes like mad if we flip on threading.  Should be we can use this to signal
it's safe to stop SkPicture::clone()ing.

[robertphillips, 2014-07-01 13:59:56]

lgtm + a question & a nit

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp
File dm/DMQuiltTask.cpp (right):

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp#newco...
dm/DMQuiltTask.cpp:44: yOffset = SkIntToScalar(fY * tile.height());
Why not just "tileCanvas.translate(-xOffset, -yOffset);" ?

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp#newco...
dm/DMQuiltTask.cpp:64: SkMutex* fMutex;  // Guards fCanvas.
INHERITED ?

[mtklein_C, 2014-07-01 14:07:18]

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp
File dm/DMQuiltTask.cpp (right):

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp#newco...
dm/DMQuiltTask.cpp:44: yOffset = SkIntToScalar(fY * tile.height());
On 2014/07/01 13:59:56, robertphillips wrote:
> Why not just "tileCanvas.translate(-xOffset, -yOffset);" ?

Good point!  Done.

https://codereview.chromium.org/360793002/diff/20001/dm/DMQuiltTask.cpp#newco...
dm/DMQuiltTask.cpp:64: SkMutex* fMutex;  // Guards fCanvas.
On 2014/07/01 13:59:56, robertphillips wrote:
> INHERITED ?

Would have, but seems silly to have it if it's not otherwise used.

[mtklein, 2014-07-01 14:14:58]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-07-01 14:16:06]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/mtklein@chromium.org/360793002/40001

[commit-bot: I haz the power, 2014-07-01 14:46:54]

Change committed as ec2ae5878d0c4fe3492b6866eaa3b946e526f1a3