Move MetadataRecorder to base/profiler and create static accessors

It's necessary to move it here from its previous location in
components/metrics because not all Chrome code is able to access code
in components/metrics due to layering restrictions. Code in
base/profiler is much more widely accessible.

As a means of testing this new location, I set a metadata item in
RenderWidgetImpl between navigation start and DidFinishLoad() and
verified that the metadata entries behaved as expected.

Bug: 913570
Change-Id: Ibf503db284119139577d613bd1214d9e3634716e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578287
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Fran├žois Doray <fdoray@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Commit-Queue: Charlie Andrews <charliea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#654217}
13 files changed