Add a UseCounter for charset replacement in XHR

Bug: 674329
Change-Id: Iac71ae590f2a94f47ae36e052b67f4ed91c4898d
Reviewed-on: https://chromium-review.googlesource.com/768309
Commit-Queue: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516277}
3 files changed