Guard ANGLE and MESA instead of defining them as native when unsupported.

Guard ANGLE and MESA instead of defining them as native when unsupported.

Since we've added "angle" to the default list of configs we've been running
kNative_GLContextType tests twice in DM, once for the real native, once for
angle.  This pointlessly doubles the GPU critical path in DM.  Whoops.

NOTREECHECKS=true

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/60c7707958a542112add1dcb246cdf9135dda870

[mtklein, 2014-08-14 16:48:49]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-08-14 16:49:10]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/mtklein@chromium.org/471063002/1

[commit-bot: I haz the power, 2014-08-14 16:49:11]

Note for Reviewers:
The CQ is waiting for an approval.
If you believe that the CL is not ready yet, or if you would like to L-G-T-M
with comments then please uncheck the CQ checkbox.

Waiting for LGTM from a valid reviewer till 2014-08-14 22:49 UTC

[bsalomon, 2014-08-14 17:35:52]

lgtm

[commit-bot: I haz the power, 2014-08-14 17:37:02]

Committed patchset #1 (1) as 60c7707958a542112add1dcb246cdf9135dda870