Add a DCHECK in Suborigin::SetName()

SetName() is never called with an empty String. This DCHECK clarifies
that to make it easy to understand that SecurityOrigin::HasSuborigin()
is equivalent to checking whether WebSecurityOrigin::Suborigin() is
empty or not.

It might also good to replace the IsEmpty() call in WebCORS.cpp with
IsNull(), but even with that, this change is just good to do, I think.

Bug: 
Change-Id: I6b49cfaa7293c8577dbec1403dff0fc58ada4fc8
Reviewed-on: https://chromium-review.googlesource.com/606757
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495117}

third_party/WebKit/Source/platform/weborigin/Suborigin.h

@@ -26,7 +26,10 @@ class PLATFORM_EXPORT Suborigin {
 
   void SetTo(const Suborigin&);
   String GetName() const { return name_; }
-  void SetName(const String& name) { name_ = name; }
+  void SetName(const String& name) {
+    DCHECK(!name.IsEmpty());
+    name_ = name;
+  }
   void AddPolicyOption(SuboriginPolicyOptions);
   bool PolicyContains(SuboriginPolicyOptions) const;
   void Clear();