Add usage counter for each value of webkit-user-modify

Add usage counter for each value of webkit-user-modify

This patch adds usage counter for each value of webkit-user-modify so
that we can have a better understanding of its current usage, and the
functionality loss about 'read-write-plaintext-only' if the property
is removed.

BUG=687843
Review-Url: https://codereview.chromium.org/2668423004
Cr-Commit-Position: refs/heads/master@{#447741}
(cherry picked from commit d71997cab809617592987ba993d0261cdde19a9a)

Review-Url: https://codereview.chromium.org/2680923004 .
Cr-Commit-Position: refs/branch-heads/2987@{#373}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/d212f25aec682c8d31ac3e09c8767e69f4718bdb

[kochi, 2017-02-08 00:34:15]

Description was changed from

==========
Add usage counter for each value of webkit-user-modify

This patch adds usage counter for each value of webkit-user-modify so
that we can have a better understanding of its current usage, and the
functionality loss about 'read-write-plaintext-only' if the property
is removed.

BUG=687843

Review-Url: https://codereview.chromium.org/2668423004
Cr-Commit-Position: refs/heads/master@{#447741}
(cherry picked from commit d71997cab809617592987ba993d0261cdde19a9a)
==========

to

==========
Add usage counter for each value of webkit-user-modify

This patch adds usage counter for each value of webkit-user-modify so
that we can have a better understanding of its current usage, and the
functionality loss about 'read-write-plaintext-only' if the property
is removed.

BUG=687843

Review-Url: https://codereview.chromium.org/2668423004
Cr-Commit-Position: refs/heads/master@{#447741}
(cherry picked from commit d71997cab809617592987ba993d0261cdde19a9a)

Review-Url: https://codereview.chromium.org/2680923004 .
Cr-Commit-Position: refs/branch-heads/2987@{#373}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/d212f25aec682c8d31ac3e09c876...
==========

[kochi, 2017-02-08 00:34:17]

Committed patchset #1 (id:1) manually as
d212f25aec682c8d31ac3e09c8767e69f4718bdb.