We want to know if a page had the autocomplete markup to enable auto fill, regardless of whether Chrome auto-filled any fields or not. This will help us encourage developers to use the autocomplete markups (if not being used) and understand how the auto fill use rate depends on the markup.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c6d68221043cb769554616d3658fd99b299024a2

commit c6d68221043cb769554616d3658fd99b299024a2
Author: csashi <csashi@google.com>
Date: Fri Mar 31 02:55:38 2017

Adds UKM for autofill attributes in form_structure.

BUG= 706646 

Review-Url: https://codereview.chromium.org/2776223002
Cr-Commit-Position: refs/heads/master@{#461005}

[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_assistant_unittest.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_experiments.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_manager.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_manager_unittest.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_metrics.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_metrics.h
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/autofill_metrics_unittest.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/form_structure.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/form_structure.h
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/form_structure_unittest.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/components/autofill/core/browser/personal_data_manager_unittest.cc
[modify] https://crrev.com/c6d68221043cb769554616d3658fd99b299024a2/tools/metrics/histograms/histograms.xml