Add ability to add annotation layers from plugins

Highlights:
* Adds a new getDiffLayers function to gr-js-api-interface.js. This is
  invoked by gr-diff-builder.html when gathering annotation layers.
* New annotationApi function in gr-public-js-api.js for plugins to call.
* The annotationApi function returns an instance of the new
  GrAnnotationActionsInterface in gr-annotation-actions-js-api.js
* GrAnnotationActionsInterface has an API for the plugin to register an
  addLayerFunction and an optional method to call to get a notify callback.
* The new samples/coverage-plugin.html is an end-to-end example of how
  to invoke the new APIs to annotate lines.

Bug: Issue 7339
Change-Id: Ie51845e0b3564953aba5d7d41986cedce0337073
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 f32234a..9da6996 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -266,6 +266,8 @@
               project-name="[[projectName]]"
               diff="[[_diff]]"
               diff-path="[[path]]"
+              change-num="[[changeNum]]"
+              patch-num="[[patchRange.patchNum]]"
               view-mode="[[viewMode]]"
               line-wrapping="[[lineWrapping]]"
               is-image-diff="[[isImageDiff]]"