Avoid using glUnmapBufferOES with ES3

As reported in https://github.com/flutter/flutter/issues/10617#issuecomment-330935921,
we were mixing ES2 and ES3 API. This makes our interface assembly
code directly parallel our caps code.

Bug: skia:
Change-Id: I91c0696a223f7eaf27641dbe16ab451b0bdc362c
Reviewed-on: https://skia-review.googlesource.com/49440
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed