Replaces the UnboundedQueue (SP/SC) with LockedQueue (MP/MC). This is necessary
as we can have multiple tasks (on multiple platform threads) reporting code move
events.

Note that the sampling queue is not affected by this change.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1457483002

Cr-Commit-Position: refs/heads/master@{#32086}