DevTools: defer reveal in TreeOutline using rAF

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=

Committed: https://crrev.com/a5f7c0a3696886422abc479b6da79c1b7dc1fbda
Cr-Commit-Position: refs/heads/master@{#361209}

[caseq, 2015-11-21 02:01:49]

caseq@chromium.org changed reviewers:
+ pfeldman@chromium.org

[pfeldman, 2015-11-21 02:04:39]

lgtm

[caseq, 2015-11-21 02:04:56]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2015-11-21 02:08:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1461283004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1461283004/1

[commit-bot: I haz the power, 2015-11-21 03:58:18]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-21 03:58:19]

Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[caseq, 2015-11-23 19:04:12]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2015-11-23 19:06:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1461283004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1461283004/1

[commit-bot: I haz the power, 2015-11-23 20:41:39]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-11-23 20:41:40]

Try jobs failed on following builders:
  linux_chromium_chromeos_rel_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)

[caseq, 2015-11-23 21:36:38]

The CQ bit was checked by caseq@chromium.org

[commit-bot: I haz the power, 2015-11-23 21:39:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1461283004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1461283004/1

[commit-bot: I haz the power, 2015-11-23 23:09:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-23 23:10:05]

Patchset 1 (id:??) landed as
https://crrev.com/a5f7c0a3696886422abc479b6da79c1b7dc1fbda
Cr-Commit-Position: refs/heads/master@{#361209}