When running angle_end2end_tests unfiltered with the OpenGL and Vulkan
backends enabled, the test window was recreated all the time and grabbed
focus every-time it was created. This made it impossible to do anything
with the machine running the tests.

Fix this by having one OSWindow per renderer group that's lazily created:
this solves most of the issue since only a couple windows end up being
created, and at the beginning of the test suite.

BUG=

Change-Id: I7a51300f0d59d8b6bb79e54d20b3acbf01068002
Reviewed-on: https://chromium-review.googlesource.com/1038433
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>