[DevTools] Add preferred size to WebInspector.View.

We now try to satisfy preferred size, but when it's impossible
ensure minimum size at least.

BUG=none

Review URL: https://codereview.chromium.org/214663005

git-svn-id: svn://svn.chromium.org/blink/trunk@170459 bbb929c8-8fbe-4397-9dbb-9b2b20218538