Implement EGL_surfaceless_context for Android/ChromeOS and optimize eglMakeCurrent BUG=angleproject:1651 Change-Id: I13c1a669d83098e22c0d7fb003d13beacc20c4ae Reviewed-on: https://chromium-review.googlesource.com/623947 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>