aura: Fix a flaky test caused by synthesized mouse-move events.

When the test window is made visible, it receives some synthesized mouse-move events
if the cursor is over the window at the time. Ignore these events.

BUG=330036
TBR=sky@chromium.org

Review URL: https://codereview.chromium.org/119613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242362 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242362