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Add a flag to save MetricKit payloads.
Saving the payload will require
- a compilation flag that will be enabled only on canary.
- setting an experimental setting

JSON representation of MetricKit payload will be saved in Documents
directory.

Collecting this data will help developing MetricKit
support in Chrome.

Bug: 1017434
Change-Id: I5db5d7112859eb873ee68e8f7159f7ab9fe3dbfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2156905
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760994}