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Up until now, there has been little help in debugging the
feature_engagement_tracker component other than seeing how the UI
behaves.

This has meant that some things are harder to evaluate and debug,
and developers typically have to add manual logging, particularly for
the check for whether in-product help should trigger.

This CL adds support for writing the ConditionValidator::Result struct
to an std::ostream using the operator<<. In addition, it makes use of
that in DVLOG(2) statements.

It also adds some extra debugging information related to startup
initialization and tracking of events.

By adding the following to the chrome command line, the extra logging
would appear in debug builds:
--vmodule=feature_engagement_tracker_impl*=2,model_impl*=2,availability_store*=2

BUG=None

Change-Id: Ifc4194d19fdec9230ad372b1d6296ffca45d5ee0
Reviewed-on: https://chromium-review.googlesource.com/526013Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#478709}