Adds formal specification for the unimplemented EGL_ANGLE_explicit_context
extension. This extension will introduce alternate entry points that allow
us to specify the context during the gl function call, so we can avoid
looking it up in TLS.

Bug: angleproject:1395
Change-Id: I6060c02b8edf586dc2443b563e2d292c00e4fbb7
Reviewed-on: https://chromium-review.googlesource.com/1033577
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>