Allow creating DartFrameIterator using explicit last_fp value (instead of always expecting it from …

runtime/vm/stack_frame.cc

Index: runtime/vm/stack_frame.cc
===================================================================
--- runtime/vm/stack_frame.cc	(revision 9960)
+++ runtime/vm/stack_frame.cc	(working copy)
@@ -170,7 +170,12 @@
   SetupLastExitFrameData();  // Setup data for last exit frame.
 }
 
+StackFrameIterator::StackFrameIterator(uword last_fp, bool validate)
+    : validate_(validate), entry_(), exit_(), current_frame_(NULL) {
+  frames_.fp_ = last_fp;
+}
 
+
 StackFrame* StackFrameIterator::NextFrame() {
   // When we are at the start of iteration after having created an
   // iterator object current_frame_ will be NULL as we haven't seen