[DevTools] Use special resizing strategy instead of insets.

This change:
- adds minimum contents size to be used in layout logic;
- switches to new API methods;
- moves layout logic into resizing strategy to avoid code duplication.

BUG=339425

Review URL: https://codereview.chromium.org/137483007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248965 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248965