Bug 1492648 - Move from nsDocShellLoadInfo to nsDocShellLoadState; r=bz, nika

Creates the nsDocShellLoadState object, which is basically
nsDocShellLoadInfo plus a few extra fields to make it usable as a
single argument to nsDocShell::LoadURI (and eventually
nsDocShell::InternalLoad).

Subframe history handling is a huge logic block in
nsDocShell::LoadURI, which is only used on history loads. This patch
also extracts the logic out into its own function to make the body of
LoadURI clearer.

docshell/base/nsDocShell.h

@@ -534,7 +534,7 @@ private: // member functions
                      uint32_t aReferrerPolicy,
                      nsIPrincipal* aTriggeringPrincipal,
                      nsIPrincipal* aPrincipalToInherit,
-                     const char* aTypeHint,
+                     const nsACString& aTypeHint,
                      const nsAString& aFileName,
                      nsIInputStream* aPostData,
                      nsIInputStream* aHeadersData,
@@ -905,6 +905,14 @@ private: // member functions
     return mCSSErrorReportingEnabled;
   }
 
+  // Handles retrieval of subframe session history for nsDocShell::LoadURI. If a
+  // load is requested in a subframe of the current DocShell, the subframe
+  // loadType may need to reflect the loadType of the parent document, or in
+  // some cases (like reloads), the history load may need to be cancelled. See
+  // function comments for in-depth logic descriptions.
+  void
+  MaybeHandleSubframeHistory(nsDocShellLoadState* aLoadState);
+
 private: // data members
   static nsIURIFixup* sURIFixup;