Don't run UnloadTest.CrossSiteInfiniteUnloadSync under TSan

Here's an example output of the failure:
[----------] 1 test from UnloadTest
[ RUN      ] UnloadTest.CrossSiteInfiniteUnloadSync
[5442:5442:0330/051539:6569997191168:INFO:chrome/test/ui/ui_test.cc(1199)] BROWSER_WRAPPER was set, prefixing command_line with /b/slave/chromium-rel-linux-tsan-ui/build/valgrind.tmp/browser_wrapper.KXl4L_
Extracting ThreadSanitizer to /tmp/valgrind.de5700
chrome/test/ui/ui_test.cc:595: Failure
Value of: tab_proxy->NavigateToURLBlockUntilNavigationsComplete( url, number_of_navigations)
  Actual: 0
Expected: AUTOMATION_MSG_NAVIGATION_SUCCESS
Which is: 1
http://mock.http/title2.html
chrome/browser/unload_uitest.cc:127: Failure
Value of: GetActiveTabTitle()
  Actual: L""
Expected: expected_title
Which is: L"Title Of Awesomeness"
[  FAILED  ] UnloadTest.CrossSiteInfiniteUnloadSync (556591 ms)
[----------] 1 test from UnloadTest (556591 ms total)

BUG=32529
TBR=glider
Review URL: http://codereview.chromium.org/1547007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@43075 0039d316-1c4b-4281-b951-d872f2087c98