Port GrGLCaps over to use SkTHash.

Port GrGLCaps over to use SkTHash.

I've written some new hashtable interfaces that should be easier to use,
and I've been trying to roll them out bit by bit, hopefully replacing
SkTDynamicHash, SkTMultiMap, SkTHashCache, etc.

This turns the cache in GrGLCaps::readPixelsSupported() into an SkTHashMap,
mapping the format key to a bool.  Functionally, it's the same.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/2aa1f7e6799501974532d35c1f6c3a0447121b95

[mtklein_C, 2015-02-20 19:41:35]

mtklein@chromium.org changed reviewers:
+ bsalomon@google.com

[bsalomon, 2015-02-20 19:50:19]

lgtm

[mtklein_C, 2015-02-20 19:51:08]

The CQ bit was checked by mtklein@chromium.org

[commit-bot: I haz the power, 2015-02-20 19:52:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/948473002/1

[commit-bot: I haz the power, 2015-02-20 19:56:41]

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

[commit-bot: I haz the power, 2015-02-20 19:56:41]

Try jobs failed on following builders:
  Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

[mtklein_C, 2015-02-20 20:04:48]

New patchsets have been uploaded after l-g-t-m from bsalomon@google.com

[mtklein, 2015-02-20 20:05:02]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-02-20 20:05:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/948473002/20001

[commit-bot: I haz the power, 2015-02-20 20:35:36]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/2aa1f7e6799501974532d35c1f6c3a0447121b95