Adds diff traversal helper and make diff object a builder property

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
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index 2e7fd56..33322df 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -151,6 +151,7 @@
           <gr-diff-builder
               id="diffBuilder"
               comments="[[_comments]]"
+              diff="[[_diff]]"
               view-mode="[[viewMode]]"
               is-image-diff="[[isImageDiff]]"
               base-image="[[_baseImage]]"