Add UMA metric for "View Original" link clicks in distilled page viewer.

components/dom_distiller/content/resources/dom_distiller_viewer.js

Index: components/dom_distiller/content/resources/dom_distiller_viewer.js
diff --git a/components/dom_distiller/content/resources/dom_distiller_viewer.js b/components/dom_distiller/content/resources/dom_distiller_viewer.js
index 3a7d0a078f2b9101073db6c1140375f9a3f4a222..9894065d4eb227e3b5fd70b170dbb0cb6880cb0e 100644
--- a/components/dom_distiller/content/resources/dom_distiller_viewer.js
+++ b/components/dom_distiller/content/resources/dom_distiller_viewer.js
@@ -65,3 +65,11 @@ var updateLoadingIndicator = function() {
     }
   };
 }();
+
+// Add a listener to the "View Original" link to report opt-outs.
+document.getElementById('showOriginal').addEventListener('click', function(e) {
+  var img = document.createElement('img');
+  img.src = "/vieworiginal";

[nyquist, 2014/08/25 01:13:03]

Would this ever show up as an ugly visual artifact for the user?

[Yaron, 2014/08/25 19:41:23]

Hmm. I suppose it's possible. Added display:none.

+  document.body.appendChild(img);

[nyquist, 2014/08/25 01:13:03]

Since the click will navigate away, could there be any timing-related issue
here, in which we would miss this beacon?

[Yaron, 2014/08/25 19:41:23]

I don't think so. Remember that the backend is always running in chrome, it's
not tied to the webpage itself being live. Adding the element to the dom should
kick off the "network" request at that moment and we'll handle it.

[nyquist, 2014/08/25 20:09:19]

Right, I think I was just concerned that the network stack would cancel
outstanding requests when you navigated away from the current page. If that is
the case, and the request for /vieworiginal hasn't hit the backend yet, we would
miss it.

But anyway, sounds good to me.

+}, true);
+