add SK_SUPPORT_LEGACY_SCALAR_XFERMODES to stage skia speedup for xfermodes

add SK_SUPPORT_LEGACY_SCALAR_XFERMODES to stage skia speedup for xfermodes

needed to enable opts in https://codereview.chromium.org/1043413002/

BUG=
TBR=fmalita@chromium.org
NOTRY=True
... cq not working :(

Committed: https://crrev.com/d9d94b9352486e833787988d909a523a8bb0e894
Cr-Commit-Position: refs/heads/master@{#323638}

[reed2, 2015-04-02 18:00:21]

reed@chromium.org changed reviewers:
+ fmalita@chromium.org, mtklein@chromium.org

[reed2, 2015-04-02 18:00:24]

The CQ bit was checked by reed@chromium.org

[commit-bot: I haz the power, 2015-04-02 18:02:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1054083002/1

[mtklein_C, 2015-04-02 18:09:14]

lgtm

[f(malita), 2015-04-02 22:04:42]

lgtm

[reed2, 2015-04-03 01:53:50]

The CQ bit was unchecked by reed@chromium.org

[reed2, 2015-04-03 01:53:56]

The CQ bit was checked by reed@chromium.org

[reed2, 2015-04-03 02:03:30]

The CQ bit was unchecked by reed@chromium.org

[reed2, 2015-04-03 02:03:39]

The CQ bit was checked by reed@chromium.org

[reed2, 2015-04-03 03:26:55]

ravi, eric, what is the delay on this CQ?

[commit-bot: I haz the power, 2015-04-03 04:41:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1054083002/1

[commit-bot: I haz the power, 2015-04-03 05:34:21]

Committed patchset #1 (id:1)

[rmistry, 2015-04-03 10:51:52]

On 2015/04/03 05:34:21, I haz the power (commit-bot) wrote:
> Committed patchset #1 (id:1)

Looks like the CQ took 2.5 hours to pick up this CL. I'll check with CQ infra
engineers if there was a known outage.

[rmistry, 2015-04-03 11:00:15]

On 2015/04/03 10:51:52, rmistry wrote:
> On 2015/04/03 05:34:21, I haz the power (commit-bot) wrote:
> > Committed patchset #1 (id:1)
> 
> Looks like the CQ took 2.5 hours to pick up this CL. I'll check with CQ infra
> engineers if there was a known outage.

Yes, there was a known outage.

[commit-bot: I haz the power, 2015-04-03 20:32:56]

Patchset 1 (id:??) landed as
https://crrev.com/d9d94b9352486e833787988d909a523a8bb0e894
Cr-Commit-Position: refs/heads/master@{#323638}