sendBeacon(): once transmission allowance has been reached, always fail.

Fix allowance checking logic for Beacon transmissions upon reaching
the limit. If the allowance limit was 'perfectly' exhausted after N
Beacon requests, subsequent Beacon requests would go ahead without
the (now zero) allowance limit imposed.

R=mkwst
BUG=701678

Review-Url: https://codereview.chromium.org/2751953002
Cr-Commit-Position: refs/heads/master@{#457088}
2 files changed