Unify tracing OWNERS with file:// indirection

TIL about file:// in OWNERS. Unifying mainteinance of
tracing OWNERS.

Review-Url: https://codereview.chromium.org/2283703002
Cr-Commit-Position: refs/heads/master@{#417308}

chrome/browser/tracing/OWNERS

-dsinclair@chromium.org
-nduca@chromium.org
-oysteine@chromium.org
-simonhatch@chromium.org
+file://base/trace_event/OWNERS

components/tracing/OWNERS

-dsinclair@chromium.org
+file://base/trace_event/OWNERS
 jbauman@chromium.org
-nduca@chromium.org
-oysteine@chromium.org
-primiano@chromium.org
-simonhatch@chromium.org

content/browser/tracing/OWNERS

-dsinclair@chromium.org
-nduca@chromium.org
-oysteine@chromium.org
-simonhatch@chromium.org
+file://base/trace_event/OWNERS