Some extensions are forced on in Context::initCaps even though the
backend's native extensions do not mark the extension as supported.
These extensions were not requestable because the
Context::isExtensionRequestable only checks if the backend supports the
extension.

Make GL_OES_vertex_array_object requestable to cover the issue.

BUG=angleproject:1523

Change-Id: Ie64df8e270924727ecf9cd3f993443abeb3ef658
Reviewed-on: https://chromium-review.googlesource.com/809197
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>