Add support for maxlength text fields to WebViewInputConnection.

 Bug 6083776
 While WebKit limits the length of a field with maxlength, the
 InputConnection can get out of sync with it when it doesn't
 recognize that the characters haven't been changed. Adds
 maximum field length to WebViewInputConnection to limit the
 characters typed.
 WebKit Change: Ie02f82a3f5b3527c378938d93bac2dece802af26

Change-Id: I135871db7809e8dc28a3ad8d3aa852976a274555