Add UMA histogram to track the various sources that request the feedback app

chrome/browser/supervised_user/supervised_user_interstitial.cc

Index: chrome/browser/supervised_user/supervised_user_interstitial.cc
diff --git a/chrome/browser/supervised_user/supervised_user_interstitial.cc b/chrome/browser/supervised_user/supervised_user_interstitial.cc
index 1671a75023230989291dad04121b3c248236eb3a..2fc4b37ba8b52d34d52c00b81e011837be235af3 100644
--- a/chrome/browser/supervised_user/supervised_user_interstitial.cc
+++ b/chrome/browser/supervised_user/supervised_user_interstitial.cc
@@ -271,7 +271,8 @@ void SupervisedUserInterstitial::CommandReceived(const std::string& command) {
     ReportChildAccountFeedback(web_contents_, message, url_);
 #else
     chrome::ShowFeedbackPage(chrome::FindBrowserWithWebContents(web_contents_),
-                             message, std::string());
+                             chrome::kFeedbackSourceSupervisedUserInterstitial,
+                             message, "" /* category_tag */);

[Lei Zhang, 2017/04/27 21:25:07]

Leave the std::string()s alone. Since ShowFeedbackPage() takes a std::string()
by const-ref, pass in a std::string to avoid the implicit construction.
Apparently that causes the compiler to generate a lot of code.

Ditto elsewhere.

[afakhry, 2017/04/27 21:39:54]

Done.

 #endif
     return;
   }