Fix some issues in the allocation tracker UI

* Sort by allocation size by default
* Don't increase number of expanded children on keyboard navigation

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/25470002

git-svn-id: svn://svn.chromium.org/blink/trunk@158604 bbb929c8-8fbe-4397-9dbb-9b2b20218538