Move the early-return case of zero mask to Context::blitFramebuffer When the mask is zero, no buffers are copied in BlitFramebuffer. But we still need to check all the error situations. So mask of zero should be put in blitFramebuffer not the validation file. Meanwhile, we adjust the checking sequence to pass the deqp case. BUG=angleproject:2375 TEST=dEQP-GLES31.functional.debug.negative_coverage.get_error.buffer.blit_framebuffer Change-Id: I73c78ffb8853153f70fc30e8ae0e704e08e454ac Reviewed-on: https://chromium-review.googlesource.com/938573 Reviewed-by: Jiawei Shao <jiawei.shao@intel.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>