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
