Replace buggy_blend_modes GM with an exhaustive test.

Replace buggy_blend_modes GM with an exhaustive test.

The new test is disabled by default, as it's quite slow.
We can run it if we suspect problems by passing -x to DM.

This test would have been failing before the bug fix, and now is passing.

Assuming the Priv on the end means it's not considered public API...
TBR=reed@google.com

BUG=skia:4052
Committed: https://skia.googlesource.com/skia/+/767d273ea0948e4f61d318bec58ce417cdc470df

[mtklein_C, 2015-07-14 21:54:06]

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

[mtklein_C, 2015-07-14 21:56:17]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-07-14 21:56:35]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1228333003/1

[commit-bot: I haz the power, 2015-07-14 22:07:30]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-14 22:07:31]

Dry run: This issue passed the CQ dry run.

[f(malita), 2015-07-14 22:27:39]

lgtm

[mtklein, 2015-07-14 22:29:34]

mtklein@google.com changed reviewers:
+ mtklein@google.com

[mtklein, 2015-07-14 22:29:35]

Just to be paranoid I'm going to wait for the bots to finish running the fixed
buggy_blend_modes so I can have a look at gold before landing this.

[mtklein, 2015-07-15 14:20:07]

On 2015/07/14 22:29:35, mtklein wrote:
> Just to be paranoid I'm going to wait for the bots to finish running the fixed
> buggy_blend_modes so I can have a look at gold before landing this.

All pending bots generated a positive image; Gold's 21-0-0.  Should be good to
land this.

Mike, do you consider SkColorPriv public API?  If so, take a look?

[mtklein, 2015-07-16 13:32:35]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-07-16 13:32:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1228333003/1

[commit-bot: I haz the power, 2015-07-16 14:01:43]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/767d273ea0948e4f61d318bec58ce417cdc470df