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Also add a function for creating an fx_logger from a LogSink.
These will be used in subsequent CLs.

Bug: 1136681,1088094
Change-Id: I9e4fe1b2b0640fa3f7eb9e0891ae9ef1a80b8bcf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2530259
Auto-Submit: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826204}