Allows extension-devtools api to survive page navigation.

The problem was that the ExtensionDevtoolsBridge maintained explicit knowledge of the RenderViewHost is was monitoring. This would be invalidated on page navigations where the inspected RVH would be swapped out from underneath it.

This patch removes knowledge of RenderViewHost from the ExtensionDevToolsBridge and makes sure it appropriately unregisters itself.

Patch by: Jaime Yap (jaimeyap@google.com)

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/348042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30840 0039d316-1c4b-4281-b951-d872f2087c98