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Crashpad maintains the client ID on its own and is responsible for
sending this form parameter to the Breakpad server during report upload.
When using Crashpad as its crash-reporting implementation, Chrome does
not need to set this crash key.

When Chrome does attempt to set this crash key on its own,
crashpad_handler produces these harmless log messages:

[mmdd/hhmmss:WARNING:crash_report_upload_thread.cc(44)] duplicate key
guid, discarding value 0123456789ABCDEF0123456789ABCDEF

There are valid reasons to provide the metrics client ID along with
the crash report, so this is placed in a distinct crash key,
"metrics_client_id".

BUG=466964
R=cpu@chromium.org, isherman@chromium.org, rsesek@chromium.org

Review URL: https://codereview.chromium.org/1000203007

Cr-Commit-Position: refs/heads/master@{#321996}