Fix broken mixed script Rappor metric and add browser test

chrome/renderer/content_settings_observer.cc

Index: chrome/renderer/content_settings_observer.cc
diff --git a/chrome/renderer/content_settings_observer.cc b/chrome/renderer/content_settings_observer.cc
index 8bf046741d65ffa0b06a4183bd22d5d53de35c2c..ac80ba79c89cbe1cc78e7b7ae7cec6e8143e5bc8 100644
--- a/chrome/renderer/content_settings_observer.cc
+++ b/chrome/renderer/content_settings_observer.cc
@@ -483,7 +483,7 @@ bool ContentSettingsObserver::allowRunningInsecureContent(
     SendInsecureContentSignal(INSECURE_CONTENT_RUN_SWF);
 
   if (!allow_running_insecure_content_ && !allowed_per_settings) {
-    DidBlockContentType(CONTENT_SETTINGS_TYPE_MIXEDSCRIPT, origin.host());
+    DidBlockContentType(CONTENT_SETTINGS_TYPE_MIXEDSCRIPT, origin.toString());

[felt, 2016/06/10 03:41:08]

Man, this is a tricky bug. It wasn't obvious to me at first what was going on
with this bug (or bugfix). I think it's so tricky for two reasons:

1. It isn't particularly clear that this string is going to end up turned back
into a GURL.
2. I would have expected TabSpecificContentSettings::OnContentBlockedWithDetail
to DCHECK that GURL(base::UTF16ToUTF8(details)).is_valid(). This would ensure
that the contract was upheld by the caller a few layers back.

I'm not suggesting that we change anything, since the bug is now found and
fixed. However: it's something I'll keep in mind the next time I see a similar
pattern!

[estark, 2016/06/10 03:55:20]

Easy enough to add a DCHECK, so I did that. Also will crib jialiul's description
of the issue that she had put in the CL she started and put it in this one, so
that the fix is a bit easier to understand.

     return false;
   }