Introduce testRunner.useSynchronousResizeMode.

This method will be used to signal that the test was written
to expect the window-resizing machinery to be synchronous,
which is not how window-resizing machinery really works.

It will be set for ~50 layout tests that were written that way.

BUG=309760
R=eseidel, jochen, ojan, tony

Review URL: https://codereview.chromium.org/34363006

git-svn-id: svn://svn.chromium.org/blink/trunk@160382 bbb929c8-8fbe-4397-9dbb-9b2b20218538