This allows us to use OS Windows and EGL contexts differently
for different implementations - eg, tests and samples.

BUG=angle:730

Change-Id: I65e69fd829c3dbf2f1b406d90045bc296798ebb6
Reviewed-on: https://chromium-review.googlesource.com/213290Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>