Disable flaky CancelationSignalTest.Cancel on iOS and Mac.

Recent flake:
http://build.chromium.org/p/chromium.mac/builders/iOS%20Simulator%20%28dbg%29/builds/15075

Flakiness dashboard:
http://chromium-build-logs.appspot.com/gtest_query?gtest_query=CancelationSignalTest.Cancel

BUG=340360
TEST=none
R=erg@chromium.org
TBR=rlarocque@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/148613010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248569 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248569