Remove unnecessary casts to MutationRecord*.

ChildListRecord, AttributesRecord, characterDataRecord, and
MutationRecordWithNullOldValue are subclasses of MutationRecord. We don't need
to cast them to the parent class.

BUG=

Review URL: https://codereview.chromium.org/111103004

git-svn-id: svn://svn.chromium.org/blink/trunk@163506 bbb929c8-8fbe-4397-9dbb-9b2b20218538