Only signal the removal callback once in CallbackList::Compact.

Noticed this typo on a whim. Right now it signals it once for every registered
callback (removed or not) after the first removed one.

BUG=none

Review URL: https://codereview.chromium.org/1420053007

Cr-Commit-Position: refs/heads/master@{#357188}
2 files changed