Make MutableStylePropertySet::m_propertyVector private

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/179233002

git-svn-id: svn://svn.chromium.org/blink/trunk@167778 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix serialization of (xlink:)href attributes in document fragments

If the namespace of an attribute has a "well-known" prefix, then make
sure that prefix is used during serialization instead of the prefix
from the attribute (which could be empty.)

BUG=345132

Review URL: https://codereview.chromium.org/177763002

git-svn-id: svn://svn.chromium.org/blink/trunk@167779 bbb929c8-8fbe-4397-9dbb-9b2b20218538