telemetry, mac: Stop using the fake keychain for telemetry tests.

The telemetry tests stopped using the real OSX keychain because the 10.9
machines were incorrectly configured. The 10.9 machines have had their
keychains fixed (Issue 397385). I've also added checks so that key Telemetry
tests log error messages if the OSX keychain is incorrectly configured
(http://code.google.com/p/chromium/issues/detail?id=443340#c3).

BUG=443340

Review URL: https://codereview.chromium.org/904163002

Cr-Commit-Position: refs/heads/master@{#315385}

tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py

@@ -167,7 +167,6 @@ class DesktopBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
     logging.info('Requested remote debugging port: %d' % self._port)
     args.append('--remote-debugging-port=%i' % self._port)
     args.append('--enable-crash-reporter-for-testing')
-    args.append('--use-mock-keychain')
     if not self._is_content_shell:
       args.append('--window-size=1280,1024')
       if self._flash_path: