[Structured metrics] Create components/metrics/structured/.

As discussed in crrev.com/c/1911067, this is where the majority of the
code for structured metrics will live, along with some initialization
in /chrome/browser/metrics.

This CL just creates the directory and adds me as OWNER. Same as for
crrev.com/c/1911067, let's revisit ownership once this is implemented.

Bug: 1016655
Change-Id: I498758fa1b7875e0bd782c32d3cc60cee7a94d33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1915640Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715458}

components/metrics/structured/OWNERS

+file://base/metrics/OWNERS
+tby@chromium.org
+
+# COMPONENT: Internals>Metrics