Don't run the second pass of the pending phantom callbacks if the heap has been torn down (Chromium issue 511204).

Debugger: prepare code for debugging on a per-function basis (issue 4132).

Performance and stability improvements on all platforms.

Cr-Commit-Position: refs/heads/4.6.20@{#1}
Cr-Branched-From: cca5e74a5814543e7446167ba47bfa498ae10c16-refs/heads/master@{#29763}