This completes the refactor to use the same code for EGL and OS
Window creation for samples and tests.

BUG=angle:730
TEST=angle_tests

Change-Id: Ib6de89f5bf83c0730a66f662cd1f87351f36a5f7
Reviewed-on: https://chromium-review.googlesource.com/213297Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>