Add an additional detail to a GCM Engine traffic annotation

As suggested by Dmitry in the previous CL:
  https://codereview.chromium.org/2737433002

BUG=656607

Review-Url: https://codereview.chromium.org/2763863003
Cr-Original-Commit-Position: refs/heads/master@{#458794}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 441c5ded961f7ae0ea52cc8f3bc5510113e9d1f2
1 file changed