[DevTools] Remove unnecessary scrollbars inside split views.

We now have exactly one View inside SplitView contents, it may have
scrollbars if needed.

BUG=340209

Review URL: https://codereview.chromium.org/152593004

git-svn-id: svn://svn.chromium.org/blink/trunk@166330 bbb929c8-8fbe-4397-9dbb-9b2b20218538