[ChromeDriver] Making CloseWindow W3C Compliant

Making CloseWindow W3C compliant by returning the result of
GetWindowHandles. Modified testCloseWindow in run_py_tests.py
to test this conformance.

Bug: chromedriver:1990
Change-Id: I2e90e9e9ca16559ea3678846a7d605b402b4d431
Reviewed-on: https://chromium-review.googlesource.com/1115813
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Cole Winstanley <cwinstanley@google.com>
Cr-Commit-Position: refs/heads/master@{#571334}
3 files changed