Convert DevToolsAgentHostRvhObserver to be a WebContentsObserver instead of a RenderViewHostObserver.

This is part of the work in removing RenderViewHostObserver.

BUG=306569
R=kaznacheev@chromium.org

Review URL: https://codereview.chromium.org/41073002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231136 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231136