Gradient shaders: make fPtsToUnit const, pre-cache getType().

Gradient shaders: make fPtsToUnit const, pre-cache getType().

This prevents races when calling fPtsToUnit.getType() from multiple threads.

This introduces a small amount of redundant code in SkTwoPointRadialGradient,
but it's probably optimized together into no extra run-time work.

BUG=437511
Committed: https://skia.googlesource.com/skia/+/cc695fee81613dc92746c1c6bb27f45cfc6ce73e

[mtklein, 2014-12-10 18:19:31]

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

[mtklein, 2014-12-10 18:19:35]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2014-12-10 18:20:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/793763003/1

[commit-bot: I haz the power, 2014-12-10 18:20:06]

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 valid reviewer(s) till 2014-12-11 00:20 UTC

[reed1, 2014-12-10 18:26:31]

lgtm

[commit-bot: I haz the power, 2014-12-10 18:29:23]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/cc695fee81613dc92746c1c6bb27f45cfc6ce73e