Bug 1177488 - use |const char*| for representing async call reasons; r=bz,fitzgen

Using a simple |const char*| is more memory-efficient than allocating a
JS string.  We still have to allocate the JS string for passing things
into JS, but ideally we will be able to move the point of allocation
much closer to where it's actually needed, rather than indiscriminantly
doing it all the time.

docshell/base/nsDocShell.cpp

@@ -14111,13 +14111,14 @@ nsDocShell::GetOpener()
   return opener;
 }
 
+// The caller owns |aAsyncCause| here.
 void
 nsDocShell::NotifyJSRunToCompletionStart(const char* aReason,
                                          const char16_t* aFunctionName,
                                          const char16_t* aFilename,
                                          const uint32_t aLineNumber,
                                          JS::Handle<JS::Value> aAsyncStack,
-                                         JS::Handle<JS::Value> aAsyncCause)
+                                         const char* aAsyncCause)
 {
   // If first start, mark interval start.
   if (mJSRunToCompletionDepth == 0) {