[DevTools] Do not recursively save sizes of all inner sidebars.

This allows user to resize outer splitview freely, while inner
splitviews accomodate to the saved preference.
For example, making DevTools window temporary very small
and then resizing it back will not reset all user-set sidebar sizes.

BUG=342649

Review URL: https://codereview.chromium.org/149473007

git-svn-id: svn://svn.chromium.org/blink/trunk@166928 bbb929c8-8fbe-4397-9dbb-9b2b20218538