* Add types to properly report what has been executed in the GC
* Unify GCPrologueCallback and GCEpilogueCallback into GCCallback
* Report processing of second round weak handels, either synchronously or asynchronously

BUG=chromium:521946
LOG=N

Review URL: https://codereview.chromium.org/1298113003

Cr-Commit-Position: refs/heads/master@{#30218}