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.
I added a warning in the `GetNativeView()`, removing it is also fine to me though.