Add Diagnostics component and self in OWNERs

Bug: 1136195
Change-Id: Ib8062352a867f256207397e20f583da609beb2cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463589
Commit-Queue: Joon Ahn <joonbug@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818016}

third_party/d3/OWNERS

 siggi@chromium.org
 chrisha@chromium.org
+joonbug@chromium.org
 
 # COMPONENT: Internals>ResourceCoordinator
+# COMPONENT: OS>Systems>Diagnostics
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