Add SourceLocationMixin and SourceLocationBase.

This allows FileLocation to avoid extending SourceLocation at all, which
avoids unused line, column, and sourceUrl fields. This produces a speed
improvement of approximately 5% in the YAML parser, and will likely do
more in code that uses locations more heavily relative to spans.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1307123004 .