Add Content-Type header to net::ReportSender reports

net/http/transport_security_state.h

Index: net/http/transport_security_state.h
diff --git a/net/http/transport_security_state.h b/net/http/transport_security_state.h
index 14ddfc00df1703cb32c3ab3b334a5647f9c44d92..eac5958bbb8e89ca9762245c35046d0b523950ef 100644
--- a/net/http/transport_security_state.h
+++ b/net/http/transport_security_state.h
@@ -252,8 +252,11 @@ class NET_EXPORT TransportSecurityState
   // An interface for asynchronously sending HPKP violation reports.
   class NET_EXPORT ReportSenderInterface {
    public:
-    // Sends the given serialized |report| to |report_uri|.
-    virtual void Send(const GURL& report_uri, const std::string& report) = 0;
+    // Sends the given serialized |report| to |report_uri| with Content-Type
+    // header as specified in |content_type|.

[eroman, 2016/09/26 21:14:07]

Can you add a note that content-type should be non-empty?
(to avoid any confusion on interpretation -- empty is not a means to omit the
content-type parameter).

[estark, 2016/09/26 23:26:10]

Done.

+    virtual void Send(const GURL& report_uri,
+                      const std::string& content_type,

[eroman, 2016/09/26 21:14:07]

What about base::StringPiece for these parameters?

I don't feel too strongly either way.
However since these are just being supplied to make a copy, StringPiece does
eliminate a redundant copy.

[estark, 2016/09/26 23:26:10]

Done.

+                      const std::string& report) = 0;
 
     // Sets a callback to be called when report sending fails.
     virtual void SetErrorCallback(