DevTools: Reveal selected placard in the callstack sidebar if needed.

- The selected placard may not be visible if navigated via keyboard shortcut
- The keydown handlers should "preventDefault" to prevent scrolling
of the parent element, i.e. use "consume(true)" instead of "consume()".

R=pfeldman

Review URL: https://codereview.chromium.org/100883005

git-svn-id: svn://svn.chromium.org/blink/trunk@163965 bbb929c8-8fbe-4397-9dbb-9b2b20218538