Pass the kGCCallbackFlagForced flag when invoking Heap::CollectAllGarbage from AdjustAmountOfExternalAllocatedMemory.

This forces the second pass of the pending phantom callbacks to run immediately after the first.

BUG=chromium:511294
LOG=Y

Review URL: https://codereview.chromium.org/1252993004

Cr-Commit-Position: refs/heads/master@{#29933}