GitLab

- Avoid copying mega bytes of feedback data when
  sending reports.
- Explicitly use OnceCallback and RepeatingCallback
  when appropriate.

BUG=748742
TEST=manual + unittests.

Change-Id: I6fb76e50f41f01a6df57e401cbfe1a1033e4db82
Reviewed-on: https://chromium-review.googlesource.com/1121077Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571947}