[pinboard] Add OWNERS file

Bug: 1029008
Change-Id: I8d91b24a8f4a593353ebafd481fd002177abb3e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1950511Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721918}

tools/perf/cli_tools/pinboard/OWNERS

+skyostil@chromium.org
+rmcilroy@chromium.org