This change moves several functions used to create test windows from WindowCaptureUtilsTest into a reusable file test_window.cc. This is so we can reuse this code in the window capturer tests in this CL: 196624: Finish implementing WGC Window Capturer and add unit tests. | https://webrtc-review.googlesource.com/c/src/+/196624 Bug: webrtc:9273 Change-Id: I62dc53e6c3475e49ca4ae4cf6d1ef02fc8781b89 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/196623 Reviewed-by: Jamie Walch <jamiewalch@chromium.org> Commit-Queue: Jamie Walch <jamiewalch@chromium.org> Commit-Queue: Austin Orion <auorion@microsoft.com> Cr-Commit-Position: refs/heads/master@{#32838}