Fixes:
 * conformance/rendering/gl-scissor-fbo-test.html
 * conformance/rendering/gl-scissor-canvas-dimensions.html

BUG=angleproject:883

Change-Id: Icd337894f1dbc7464a4566bfab9a30b70fd4bbfa
Reviewed-on: https://chromium-review.googlesource.com/274445Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>