Adds a polymorphic method to GrDiffBuilder subclasses named
`_getNextContentOnSide` which gets the a content element by traversing
from its preceding content on the same side. This method is dramatically
faster than the query-based method when diff sections are large.
In preparation for the syntax highlighting change, the gr-diff-builder
is refactored to use a property for the diff object, rather than
accepting it as a parameter to the `render` function.
Tests are included for new functions.
Change-Id: Ifd0edb530a303de2626d55a691c3ba1eaed6167c
ea97c5b49a