Add preference for line wrapping in diff preferences

Previously in Polygerrit, diff views were always displayed in the width
specified in diff preferences. This change gives the option to wrap
lines instead, which takes precedence over column width (the column
width option is hidden when line wrapping is selected), and fits the
diff view to screen.

The gerrit API already supports the 'lineWrapping' preference so this
change uses that already existing option.

Feature: Issue 4809
Change-Id: I0d9e292739b5910abfd04af63ec4c745bf06e446

polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_test.html

@@ -546,6 +546,7 @@ limitations under the License.
                   assert.isTrue(scrollStub.called);
                   document.body.style.height =
                       originalHeight + 'px';
+                  scrollStub.restore();
                   done();
                 });
             document.body.style.height = '10000px';