Disable flaky PPAPINaClGLibcTest.URLRequest_AppendFileToBody.

Failing on Linux: http://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/3186

Flakiness dashboard: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Chromium&testType=browser_tests&showExpectations=true&tests=PPAPINaClGLibcTest.URLRequest_AppendFileToBody%2CPPAPINaClGLibcTest.URLLoader%2CPPAPINaClGLibcTest.FileIO%2CPPAPINaClGLibcTest.FileRef

BUG=307949
TBR=yzshen

Review URL: https://codereview.chromium.org/27476006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228897 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228897