Use a new class name for each window.

If a previous window was not cleaned up properly due to a crash in a
test, the subsequent windows can fail to be created because the class is
not destroyed.  Work around this by always creating a unique class before
creating the window.

Change-Id: Ied6b2818ef03fa12b07111d8204c3c1a6a5bd5ac
Reviewed-on: https://chromium-review.googlesource.com/225080
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>