MarkupAccumulator: Introduce a new class representing two namespace maps.

Store PrefixToNamespaceMap and NamespaecToPrefixesMap in a single class.
This CL has no behavior changes.

Bug: 927166
Change-Id: I9c04af6b16961bdba29177417b2b7ec75fbb0188
Reviewed-on: https://chromium-review.googlesource.com/c/1469757
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631617}
2 files changed