Disable WebViewTest.Shim_TestDeclarativeWebRequestAPI on Mac because it's flaky on trybots.

BUG=326330
TBR=fsamuel@chromium.org

Review URL: https://codereview.chromium.org/106983003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239061 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239061