gr-diff-builder.html and gr-diff-builder.js didn't follow convention
about naming. The .html file contained <gr-diff-builder> element and a
js code for it. The gr-diff-builder.js file contains some other code.
Polymer modulizer can't update such files.
This change renames gr-diff-builder.html to gr-diff-builder-element.html
and moves javascript to a separate file.
Change-Id: Ic25396de8e98a528806b740323bce4c5b3507d3d
19736a58c3