Expose OES_framebuffer_object extension for GLES1 contexts.

Bug: angleproject:5401
Test: FramebufferObjectTest*
Change-Id: I63b504f580cdab4655b4bfbd3438bcd807b422ea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2561795
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>