EXO: Change Surface before committing in fullscreen shell test

This fixes a DCHECK caused by an empty output surface rect since there
is no change in the Surface when comitting.

Bug: 896710
Test: Run exo_unittests on Chromecast with debug build
Change-Id: I661a1ca206a3f0e9a607d4afdcd4680e4fa76c7c
Reviewed-on: https://chromium-review.googlesource.com/c/1345241
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614142}
1 file changed