We can concatenate all possible "labels" (<label>, text around, placeholder, aria-label, etc.) to |FormFieldData.label|. The more info we have, the more chances to find a text feature. 

Use concatenation instead of assignment in |FormOrFieldsetsToFormData|, |MatchLabelsAndFields| and |InferLabelForElement|. 

rogerm@chromium.org, mathp@chromium.org, isherman@chromium.org: how do you feel about #1?

Agreed, this is worth doing and a good first bug if we don't get around to doing it sooner.

sebsg@, michaelbai@: FYI, we are going to concatenate labels. any objections? 

Sebastien: longer labels may slow down crowdsourcing. Hopefully not much. If is it an issue, we consider more sophisticated algorithms (say, in 75% of uploads do upload the first 25 characters). 
Positive side is that concatenation can improve client-side username detection.

Sounds good!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f5dc74a384d48f1a0929dc056cadae2a0019f8b5

commit f5dc74a384d48f1a0929dc056cadae2a0019f8b5
Author: Luka Dojcilovic <l.dojcilovic@gmail.com>
Date: Thu Mar 08 23:23:10 2018

[Autofill] Added aria-label to InferLabelForElement

Added aria-labels as one of the possible things to be used as a label
for form control elements because they may contain useful information
for username detection.

Bug:  783185 
Change-Id: I59a7f025c616f38d533c9c9a7327ec8ec9021c4c
Reviewed-on: https://chromium-review.googlesource.com/951765
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sebastien Seguin-Gagnon <sebsg@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541929}
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/content/common/autofill_types.mojom
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/content/common/autofill_types_struct_traits.cc
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/content/renderer/form_autofill_util.cc
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/content/renderer/form_autofill_util_browsertest.cc
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/core/common/form_field_data.h
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/autofill/ios/fill/resources/fill.js
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/test/data/autofill/heuristics/input/117_cc_checkout_macys.com.html
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/test/data/autofill/heuristics/output/117_cc_checkout_macys.com.out
[modify] https://crrev.com/f5dc74a384d48f1a0929dc056cadae2a0019f8b5/components/test/data/autofill/heuristics/output/125_bug_462080.out