Add commandline switch to enable DTLS 1.2 for WebRTC.

This CL adds the commandline switch "--enable-webrtc-dtls12" to enable
negotiation of DTLS 1.2 for WebRTC peerconnections.

BUG=428343

Review URL: https://codereview.chromium.org/1157093007

Cr-Commit-Position: refs/heads/master@{#332381}

• AUTHORS[diff]
• chrome/app/generated_resources.grd[diff]
• chrome/browser/about_flags.cc[diff]
• content/browser/renderer_host/render_process_host_impl.cc[diff]
• content/public/common/content_switches.cc[diff]
• content/public/common/content_switches.h[diff]
• content/renderer/media/webrtc/peer_connection_dependency_factory.cc[diff]
• tools/metrics/histograms/histograms.xml[diff]