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There is already some debug output available for origin trials,
which can be enabled with:

 % chrome --vmodule=origin_trial_context=1 --enable-logging=stderr

This CL adds a bit more human-readable debug output, plus it adds
the appropriate flags to the documentation, so it is easier to find.

TL/DR now:
 % chrome --vmodule=trial_token=2,origin_trial_context=1 --enable-logging=stderr

Change-Id: I27d08055f460287e02531886593c696e700bbe3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347385Reviewed-by: Jason Chase <chasej@chromium.org>
Commit-Queue: Mason Freed <masonfreed@chromium.org>
Auto-Submit: Mason Freed <masonfreed@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799308}