[DevTools] Make all split views use dip.

Now all split views retain their position on zoom changes.
Split view also operates/stores with sizes in dip, except for DOM modifications,
where css pixels are required.

Making all split views zoom-aware has a positive side effect of not breaking
minimum-size relations when zooming.

BUG=none

Review URL: https://codereview.chromium.org/197273002

git-svn-id: svn://svn.chromium.org/blink/trunk@169350 bbb929c8-8fbe-4397-9dbb-9b2b20218538