Fix use of multiple raster threads for software rendering.

Fix use of multiple raster threads for software rendering.

By defaulting Threaded GPU Rasterization to ON I inadvertently
disabled using more than 1 raster thread for Software rasterization
everywhere.

Fixing so IsThreadedGpuRasterizationEnabled() is only on if Gpu
rasterization is enabled or forced, so this will currently only be
on for Android.

We'll need a better way to support multiple raster worker threads
to turn on GPU rasterization on desktops, or to support multiple render
threads on Android.

BUG=454500
BUG=459760
Committed: https://crrev.com/f6ee6a2238d1ba7032a09d335cfc9a9d5d870232
Cr-Commit-Position: refs/heads/master@{#316949}

[vmiura, 2015-02-18 22:23:02]

vmiura@chromium.org changed reviewers:
+ piman@chromium.org, sievers@chromium.org

[vmiura, 2015-02-18 22:23:03]

PTAL

[no sievers, 2015-02-18 22:40:41]

lgtm

[reveman, 2015-02-18 22:45:24]

reveman@chromium.org changed reviewers:
+ reveman@chromium.org

[reveman, 2015-02-18 22:45:24]

lgtm

[piman, 2015-02-18 23:06:07]

lgtm3

[piman, 2015-02-18 23:06:15]

lgtm3

[vmiura, 2015-02-18 23:43:00]

The CQ bit was checked by vmiura@chromium.org

[commit-bot: I haz the power, 2015-02-18 23:44:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/941443002/20001

[commit-bot: I haz the power, 2015-02-19 01:11:59]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-19 01:12:31]

Patchset 2 (id:??) landed as
https://crrev.com/f6ee6a2238d1ba7032a09d335cfc9a9d5d870232
Cr-Commit-Position: refs/heads/master@{#316949}