Added RTP data channel checkbox to peerconnection.html

BUG=279595
TEST=Manual tests on the peerconnection.html page and WebRTC browser tests:
WebrtcApprtcBrowserTest.MANUAL_WorksOnApprtc
WebrtcAudioQualityBrowserTest.MANUAL_TestAudioQuality
WebrtcBrowserTest.MANUAL_CpuUsage15Seconds 
WebrtcBrowserTest.MANUAL_RunsAudioVideoCall20SecsAndLogsInternalMetrics
WebrtcBrowserTest.MANUAL_RunsAudioVideoWebRTCCallInTwoTabs
WebrtcBrowserTest.MANUAL_TestMediaStreamTrackEnableDisable
WebrtcVideoQualityBrowserTest.MANUAL_TestVGAVideoQuality 

NOTRY=TRUE

Review URL: https://codereview.chromium.org/27168002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228674 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228674