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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1de1f550d98da0b96db3838564696fede189392e

commit 1de1f550d98da0b96db3838564696fede189392e
Author: Sujie Zhu <sujiezhu@google.com>
Date: Thu Sep 06 16:23:05 2018

[Local card migration] add local card migration prompt metrics

We add one metric enum to represent each window shown/acceptance and one enum to denote the migration triggering source.

We record the histogram of each window shown/acceptance for each kind of the triggering source, which means for each entry point, we create a separate sub-histogram for the metric.

Bug:  880871 
Change-Id: I2f1ac0561dcf44e81d8138b5250c68bf3b697be4
Reviewed-on: https://chromium-review.googlesource.com/1197326
Reviewed-by: Jared Saul <jsaul@google.com>
Reviewed-by: Robert Kaplow (slow) <rkaplow@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#588859}(cherry picked from commit 2e8c7f72e614e51096d6592acf1fdcc6a7700a62)
Reviewed-on: https://chromium-review.googlesource.com/1210547
Cr-Commit-Position: refs/branch-heads/3538@{#90}
Cr-Branched-From: 79f7c91a2b2a2932cd447fa6f865cb6662fa8fa6-refs/heads/master@{#587811}
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/components/autofill/core/browser/autofill_metrics.cc
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/components/autofill/core/browser/autofill_metrics.h
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/components/autofill/core/browser/local_card_migration_manager.cc
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/components/autofill/core/browser/local_card_migration_manager.h
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/components/autofill/core/browser/local_card_migration_manager_unittest.cc
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/1de1f550d98da0b96db3838564696fede189392e/tools/metrics/histograms/histograms.xml