Add username field discovery heuristic.

Add username field discovery heuristic.

There are some cases when autofill can't find a username field for
autocompletion. For example, if the field is created dynamically in JavaScript.
This CL introduces a heuristic which is used as a fallback in such cases to
find a username field.

When password form autocompletion is about to happen with unknown username
field, the heuristic searches for the closest visible autocompletable
non-password text element preceding the password element. If one found, it is
believed to be the username field.

BUG=710374, 606736, 586465, 710003
Review-Url: https://codereview.chromium.org/2889393002
Cr-Original-Commit-Position: refs/heads/master@{#475481}
Review-Url: https://codereview.chromium.org/2918543002 .
Cr-Commit-Position: refs/branch-heads/3112@{#52}
Cr-Branched-From: b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed: https://chromium.googlesource.com/chromium/src/+/525e06d30faf3fdbfb85e2b0e1e7c0c1bdc031e5

[pkalinnikov, 2017-05-31 13:49:53]

Description was changed from

==========
Add username field discovery heuristic.

There are some cases when autofill can't find a username field for
autocompletion. For example, if the field is created dynamically in JavaScript.
This CL introduces a heuristic which is used as a fallback in such cases to
find a username field.

When password form autocompletion is about to happen with unknown username
field, the heuristic searches for the closest visible autocompletable
non-password text element preceding the password element. If one found, it is
believed to be the username field.

BUG=710374,606736,586465,710003

Review-Url: https://codereview.chromium.org/2889393002
Cr-Commit-Position: refs/heads/master@{#475481}
(cherry picked from commit 4afdcc3b649fd9e056ccfe078890c39419e62d08)
==========

to

==========
Add username field discovery heuristic.

There are some cases when autofill can't find a username field for
autocompletion. For example, if the field is created dynamically in JavaScript.
This CL introduces a heuristic which is used as a fallback in such cases to
find a username field.

When password form autocompletion is about to happen with unknown username
field, the heuristic searches for the closest visible autocompletable
non-password text element preceding the password element. If one found, it is
believed to be the username field.

BUG=710374,606736,586465,710003

Review-Url: https://codereview.chromium.org/2889393002
Cr-Original-Commit-Position: refs/heads/master@{#475481}
Review-Url: https://codereview.chromium.org/2918543002 .
Cr-Commit-Position: refs/branch-heads/3112@{#52}
Cr-Branched-From:
b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed:
https://chromium.googlesource.com/chromium/src/+/525e06d30faf3fdbfb85e2b0e1e7...
==========

[pkalinnikov, 2017-05-31 13:49:55]

Committed patchset #1 (id:1) manually as
525e06d30faf3fdbfb85e2b0e1e7c0c1bdc031e5.