Hook intervention reports up to Reporting API

This patch connects intervention reports to the ReporingServiceProxy mojo

Notes on testing: 
1) The test for this patch's new functionality is added to reporting-api.html,
where the mock reporting class there can be reused.
2) This patch also factors out the intervention-generating code to a
function in intervention.js, so it can be used in both reporting-api.html
and intervention.html tests.

Bug: 731810, 564071
Change-Id: I9ea961f6672ece81712c657ee7d9350b0520bb29
Reviewed-on: https://chromium-review.googlesource.com/685992
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Paul Meyer <paulmeyer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504850}
6 files changed