Generate list of GPU contexts outside ImageTest tests

Generate list of GPU contexts outside ImageTest tests

Use DEF_GPUTEST_FOR_*_CONTEXT macros to obtain the
test GPU context.

Makes changing the context -related classes easier,
since not all tests need to be changed.

BUG=skia:2992
Committed: https://skia.googlesource.com/skia/+/7b94c1479fc4ab3b7526808ac83c73f48d3cd721

[Kimmo Kinnunen, 2015-11-17 10:16:42]

kkinnunen@nvidia.com changed reviewers:
+ bsalomon@google.com

[bsalomon, 2015-11-19 15:31:21]

lgtm

[Kimmo Kinnunen, 2015-11-24 14:06:15]

The CQ bit was checked by kkinnunen@nvidia.com

[commit-bot: I haz the power, 2015-11-24 14:06:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1454663002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1454663002/1

[commit-bot: I haz the power, 2015-11-24 14:08:39]

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

[commit-bot: I haz the power, 2015-11-24 14:08:40]

Try jobs failed on following builders:
  Build-Mac10.8-Clang-x86_64-Release-Trybot on client.skia.compile (JOB_FAILED,
http://build.chromium.org/p/client.skia.compile/builders/Build-Mac10.8-Clang-...)

[Kimmo Kinnunen, 2015-11-24 15:26:42]

The CQ bit was checked by kkinnunen@nvidia.com

[Kimmo Kinnunen, 2015-11-24 15:26:42]

The patchset sent to the CQ was uploaded after l-g-t-m from bsalomon@google.com
Link to the patchset: https://codereview.chromium.org/1454663002/#ps20001
(title: "fix mac build")

[commit-bot: I haz the power, 2015-11-24 15:26:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1454663002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1454663002/20001

[commit-bot: I haz the power, 2015-11-24 15:39:42]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/7b94c1479fc4ab3b7526808ac83c73f48d3cd721