Make the window one pixel narrower in thumbnail test.

Some ARM chromebooks do not support running two fullscreen apps at the
same time. If chrome is running, and we run the thumbnail test, the test
will hang because the two apps try to flip at the same time.

The workaround is to make the thumbnail test window a pixel narrower,
so it is not fullscreen.

BUG=chrome-os-partner:23314
TEST=run the test and it does not hang

Review URL: https://codereview.chromium.org/27681002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229618 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229618