DevTools: defer reveal in TreeOutline using rAF

- only do scrollIntoViewIfNeeded() in a rAF callback to avoid forcing layout;
- get rid of TreeElement.onreveal() as most implementation were a no-op just
    repeating what the call site has already done;
- added `center' parameter to TreeElement.reveal()
- do not center when scrolling with keyboard or searching;

BUG=

Review URL: https://codereview.chromium.org/1461283004

Cr-Commit-Position: refs/heads/master@{#361209}