Oilpan: Add more trace events to platform/heap/

I want to keep track of time spent in the methods.

BUG=420515

Review URL: https://codereview.chromium.org/646553005

git-svn-id: svn://svn.chromium.org/blink/trunk@183529 bbb929c8-8fbe-4397-9dbb-9b2b20218538