[DevTools] Remove window onscroll handler and use margins instead.

Scroll handler is unreliable, while negative margin does the work.

BUG=none

Review URL: https://codereview.chromium.org/396663002

git-svn-id: svn://svn.chromium.org/blink/trunk@179476 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Simplify TracedValue

This change makes TracedValue interface similar to the one in Chromium. Also a unit test is added.

BUG=361045

Review URL: https://codereview.chromium.org/436673006

git-svn-id: svn://svn.chromium.org/blink/trunk@179477 bbb929c8-8fbe-4397-9dbb-9b2b20218538