Place DevTools WebContents underneath inspected WebContents.

DevTools window: split width and height are gone, and instead
contents insets are introduced. These insets are used for layout
in browser window. Having insets instead of rect allows for faster
regular-case resize occuring immediately in layout:
we keep DevTools of the same size and resize the page contents.

Views: removed SplitView and ContentsContainer, but instead added
ContentsLayoutManager which layouts DevTools and page contents taking
insets into account. It also supports active top margin, moved from
ContentsContainer.

Mac: removed NSSplitView, but instead added DevToolsContainerView
which resizes subviews manually taking page insets into account.

Gtk: removed both contents_vsplit and contents_hsplit, but instead
added devtools_floating_container which positions page contents
taking insets into account.

Next step will remove dock side knowledge from the browser, and leave it
on frontend only.

BUG=318751

Review URL: https://codereview.chromium.org/63173016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241223 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241223