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When working on out-of-process mojo services it is hard to figure out
which log line comes from which process. Often both the browser process
and the remote process are running shared components (e.g. code in
//ui/views in both out-of-process ash and browser process). Trying to
back-correlate pids with process types is really awkward.

Add a optional prefix to the start of log lines so things look like:
[my_process:80672:80672:0730/151208.285440:ERROR:file.cc(2982)]...

Only add the prefix to new mojo service processes because it's
possible there are consumers of existing browser process logging
and I don't want change the format of existing browser or renderer
logs.

Bug: 869181
Test: added to base_unittests
Change-Id: Ia26a78d69c19e1f5cbf8ecd269e962ad0589d008
Reviewed-on: https://chromium-review.googlesource.com/1156988Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579918}