Rename GrContextFactory::ContextOptions to ContextOverrides

Rename GrContextFactory::ContextOptions to ContextOverrides

Also changes the behavior of these flags to only override their
corresponding context options when set, and to leave them unchanged
when not set.

BUG=skia:

Change-Id: I09f6be09997594fa888d9045dd4901354ef3f880
Reviewed-on: https://skia-review.googlesource.com/8780
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>