This makes us use std::ostringstream in more places, instead of
string concatenation.

BUG=chromium:697758

Change-Id: Ifdcaa2e7e119664fc9cfdc566ea13b519a294714
Reviewed-on: https://chromium-review.googlesource.com/521729
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>