VR: Implement testapp window resizing

The testapp window now has the proper aspect ratio, initially and after
resizing the window.

BUG=

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ie83f6fc769da7c44d9e9b140743645532d8f3192
Reviewed-on: https://chromium-review.googlesource.com/782061


Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518292}