Add support for GL_NV_fence.

Will be used for YUV420 readback on ANGLE ES 2 contexts in Chrome.

Marks the functions as required as Chrome now adds inits these on
GrGLInterface.

Bug: 1040643
Change-Id: I5504d4c9209874991592c9f86aaf7987c316aa40
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265602
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed