DevTools: constrain zoom and offsets in Layers & Paint Profiler views

Make sure some of the image remains visible in the viewport.

BUG=426096

Review URL: https://codereview.chromium.org/664663003

git-svn-id: svn://svn.chromium.org/blink/trunk@184440 bbb929c8-8fbe-4397-9dbb-9b2b20218538