Don't allow blitFramebuffer dimensions to overflow 32-bit integer
range as specified in WebGL 2.0 section 5.41.

BUG=chromium:830046
TEST=WebGL 2 conformance tests, angle_end2end_tests

Change-Id: Ia232291b09c94e1e4f837441c6720a78bab672fb
Reviewed-on: https://chromium-review.googlesource.com/1023856Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>