Rewrite the devtools view for Chromium's new devtools architecture

* Refactors the docked devtools implementation by overlapping contents view and devtools view instead of using native split view, see https://code.google.com/p/chromium/issues/detail?id=318751.
* Uses `views` to implement docked and undocked devtools on both Linux and Windows.
:+1: for using `scoped_ptr` here to make the ownership explicit. (I wish `RegisterDictionaryPref` took a `scoped_ptr`!).