…an attribute with an existing namespace

Add an implementation of "retrieving a preferred prefix string"
[1], and apply it to attribute serialization.

As the specification defines, this CL introduces another map [2] like:

  namespace URI => [older prefix, ..., newer prefix]

in order to retrieve most-recently-declared prefix.

When we search the map for a prefix, we check availability of
the prefix by the existing prefix-namespace map.

There are two failing test cases in XMLSerializer-serializeToString.html.
They follow the specification.
  * The first failing one: An non-well-formed XML is produced if we follow
    the specification. Chrome's new behavior matches to Firefox. Safari also
    produces a bad XML.
  * The second failing one: Firefox, Safari, and Chrome without this CL
    preserve prefix in such case.  This CL keeps the interoperable behavior.

[1] https://w3c.github.io/DOM-Parsing/#dfn-retrieving-a-preferred-prefix-string
[2] https://w3c.github.io/DOM-Parsing/#dfn-namespace-prefix-map

Bug: 906807
Change-Id: Idaea590ba19c9c21acbaf456ea91ce550c74db5a
Reviewed-on: https://chromium-review.googlesource.com/c/1445033
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631573}