interactive tests: Complete in-progress animations before sending mouse events.

Sending mouse events while it animates can be flaky, so stop any in-progress
animation of the window before sending mouse events to it.

BUG=none
R=sky@chromium.org

Review URL: https://codereview.chromium.org/138633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244821 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244821