This was a missing part of the validation. Makes us pass the WebGL
test conformance2/rendering/blitframebuffer-outside-readbuffer.

Was necessary for lazy robust resource init.

BUG=angleproject:2107
BUG=chromium:644740

Change-Id: I54c50012fc09ec80a65a2e75f5bde05101c8a1a7
Reviewed-on: https://chromium-review.googlesource.com/663212Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>