Disable LocalFileSyncContextTest.PrepareSyncWhileTesting.

This test is flaky when running on the TSAN v2 bot and causes otherbots to hang.

NOTRY=true
R=kinuko@chromium.org
TBR=kinuko@chromium.org
BUG=305905

Review URL: https://codereview.chromium.org/26524003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227881 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227881