[DevTools] Limit responsive design resize request to width/height only.

Before, if we had not enough space and clipped the page height,
resizing width will also reset height to the clipped size.

BUG=none

Review URL: https://codereview.chromium.org/321513004

git-svn-id: svn://svn.chromium.org/blink/trunk@175687 bbb929c8-8fbe-4397-9dbb-9b2b20218538