gpu: Fix a bug in the generation of the destruction sync token for GMBs.

This bug was recently introduced (https://codereview.chromium.org/1499813003).
It created a race condition that would cause GMBs to be used after they were
destroyed, since the destruction sync token wasn't being properly constructed.

BUG=514815

Review URL: https://codereview.chromium.org/1525273003

Cr-Commit-Position: refs/heads/master@{#365437}