Revert 230554 "Adds a browser test to verify that the DTLS certi..."

The test often times out:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%2540ToT%2520Chromium&testType=content_browsertests&tests=WebrtcBrowserTest.DtlsCertificateCached
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=content_browsertests&group=%40ToT%20Blink&tests=WebrtcBrowserTest.DtlsCertificateCached

> Adds a browser test to verify that the DTLS certificate is cached.
> 
> BUG=304228
> 
> Review URL: https://codereview.chromium.org/26581005

TBR=jiayl@chromium.org

Review URL: https://codereview.chromium.org/39513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230665 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230665