Refactor JavaScriptFrame::function() to return a JSFunction* and remove associated casts.

src/runtime-profiler.cc

Index: src/runtime-profiler.cc
diff --git a/src/runtime-profiler.cc b/src/runtime-profiler.cc
index bd02a6904270c9fadfb0766fb755a86141407e37..ff41432b284a4c3a2d9ea9231b51b5ca5dd8c752 100644
--- a/src/runtime-profiler.cc
+++ b/src/runtime-profiler.cc
@@ -247,7 +247,7 @@ void RuntimeProfiler::OptimizeNow() {
        frame_count++ < frame_count_limit && !it.done();
        it.Advance()) {
     JavaScriptFrame* frame = it.frame();
-    JSFunction* function = JSFunction::cast(frame->function());
+    JSFunction* function = frame->function();
 
     if (!FLAG_watch_ic_patching) {
       // Adjust threshold each time we have processed