A number of classes used in PolyGerrit are defined in vanilla JS files
that are included multiple times by various elements. For example,
gr-diff-line.js is included by the gr-diff-builder and by gr-diff
elements. Adds #ifndef-style guards to each of these libraries to
prevent redefinition and avoid different elements potentially referring
to different versions of the same class.
Change-Id: I45e3ba425a59989b328475b1fe58fd9f350c8ae0
16810999f6