Reland "Add UMA for sync favicons in history WebUIs"

This is a reland of 19d0ffa55117052243499e0216e56aef6ab0eda3

Original change's description:
> Add UMA for sync favicons in history WebUIs
>
> We add metrics to evaluate sync's usefulness in helping obtain favicons
> for chrome://history and chrome://history/syncedTabs that would not be
> available locally.
>
> Bug: 948109
> Change-Id: I38b527ffd3c0efb9c452d84e1c44c465b1234cec
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1561112
> Commit-Queue: Victor Vianna <victorvianna@google.com>
> Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
> Reviewed-by: Dan Beam <dbeam@chromium.org>
> Reviewed-by: Marc Treib <treib@chromium.org>
> Reviewed-by: Brian White <bcwhite@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#652116}

TBR=bcwhite@chromium.org

Bug: 948109
Change-Id: I233641c6c44e237227ff305cfdc159f5ced912df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572411
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653130}
7 files changed