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Chromium's log output to the Fuchsia logging service has a lot of
header boilerplate because each log line has concatenated prefixes
from both Chromium and Fuchsia. This makes syslog data hard to read
because each line is very likely to wrap around on an 80-100col
terminal.

This CL strips the Chromium logging prefix from the logging data
and provides log lines' source location as log "tags".

Sample log line prefix, before this CL (146 chars):
[00582.348425][130450][130452][ui_base_unittests__exec] ERROR: \
[130450:3944750047:1007/220315.685215:582348409:ERROR:base_paths_fuchsia.cc(21)] ...

Sample log line prefix, after this CL (89 chars):
[00517.527091][108865][108867][ui_base_unittests__exec] ERROR: base_paths_fuchsia.cc(21) ...

Bug: 1012037
Change-Id: Ia51442711ca8575246170b5090a4abafc45fcc43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846395
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Auto-Submit: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705656}