Disable flaky RootWindowTest.TouchMovesHeld on ChromeOS.

RootWindowTest.TouchMovesHeld is flakily failing on CrOS Valgrind bots.
The cause is unclear, but it might be related to r230206 or r244436.
Disable the test on ChromeOS for now to allow for followup investigation.

BUG=333644
TEST=NONE
TBR=thakis@chromium.org,tdresser@chromium.org
NOTRY=true
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/136013002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244449 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244449