A new method is added to the GrDiffBuilder class named
`_renderContentByRange`. It will replace DIV.contentText elements with
newly-rendered versions for the given line-range and side.
Two utility methods found on GR-DIFF-BUILDER are pushed down into the
GrDiffBuilder class. In particular, `getContentByLine` is moved as-is
and a wrapper is added to original place. `getContentsByLineRange` is
moved and converted to be more general. A wrapper is put in its original
location which follows the original signature. These moves make the
functions available to other methods of the class.
Tests are added for these new and existing methods.
Change-Id: I77634d05828756c46b802f9ec789ab767faca3cf
9317bafd74