[Onion Soup] Move webrtc_uma_histograms.h|cc from content to Blink

Header file initially in public/platform so that it can still be
accessed by dependents in content.
Once dependents are migrated to blink, it will be moved to
renderer/platform.

Bug: 704136
Change-Id: If66f57d7239fe585a77d07a098ad20dd51d8e4f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1499553
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637613}
17 files changed