ANGLETest: Reuse test windows per-renderer 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>