Fix being unable to request some extensions implemented in the GL layer. 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>