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This CL implements the ability to divide a label between a series of
fields to address the following scenario in a form:

Street Name / House Number: [             ][  ]

or

First Name & Last Name: [           ][           ]

([       ]: indicates a text input field in a form.)

Here, it is anticipated that the label is divided between the two
following fields.

For this to happen, the label is split by a list of separator characters
and words. If the field is followed by a series of unlabeled fields such
that the total number of fields exactly matches the non-empty components
of the first fields' label after the split, the label is divided.

There are additional conditions for the label sharing:
* The maximum number of fields to share a label is limited to 3.
* The maximum length of the label to be shared is limited to 40.

To store the divided labels, a new field |parseable_label_| is added to
|AutofillField| which is only supposed to be used in the heuristic
type detection.

Note, this change only makes the |parseable_label_| available,
but does not utilize this yet. This will be subject to a subsequent CL.

Change-Id: If098b2aa5a0c3c29a28fff846a4c694c64b837f5
Bug: 1167702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623547Reviewed-by: Christoph Schwering <schwering@google.com>
Commit-Queue: Matthias Körber <koerber@google.com>
Cr-Commit-Position: refs/heads/master@{#844596}