Autofill: Port r310631 to iOS.

Autofill: Port r310631 to iOS.

https://crrev.com/310631: improve handling of nameless, ID-less inputs

This port is a bit different from r310631 in that it uses an array to
hold the extracted fields instead of a map. The issue is that JS can
only use strings or numbers as the key to dictionaries, and not web
elements. Web Elements will have their toString() method called, so they
all occupy the same key.

To get around this, one potential solution is to use Map from ECMA 6,
but iOS 7 does not support Map, so that is a no go.

BUG=427614

Review URL: https://codereview.chromium.org/1158333003

Cr-Commit-Position: refs/heads/master@{#334345}