Handles of this type are automatically reset by the garbage collector
when their objects are not longer reachable.

The motivation is to reduce pause time of external.weak_global_handles
phase of the garbage collector by not maintaing the list of pending
callbacks and not calling the callbacks.

Local testing on discourse page of the v8.inifinite_scroll benchmark
shows 7x improvement for this GC phase.

Before:
external.weak_global_handles
 len: 21
 min: 0.0
 max: 4.5
 avg: 0.757142857143

After:
external.weak_global_handles
 len: 21
 min: 0.0
 max: 0.5
 avg: 0.109523809524

A follow-up patch will enable the new phantom handles in Chromium.

BUG=chromium:608333
LOG=NO

Review-Url: https://codereview.chromium.org/1950963002
Cr-Commit-Position: refs/heads/master@{#36095}