move skia metrics helpers to SkFont prefix

follow-on to CL 1343038

Bug: skia:2664
Change-Id: I5ae5640b6c8b025ba531bec633b71484cf4249d1
Reviewed-on: https://chromium-review.googlesource.com/c/1351371
Commit-Queue: Mike Reed <reed@google.com>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610886}
5 files changed