Change root visitor to use Object**.

Simplifies code and improves the performance of root visiting since
we usually don't need to check to see if the object moved.

Change-Id: Iba998f5a15ae1fa1b53ca5226dd2168a411196cf
diff --git a/runtime/arch/context.h b/runtime/arch/context.h
index 3d11178..83bbb11 100644
--- a/runtime/arch/context.h
+++ b/runtime/arch/context.h
@@ -49,6 +49,9 @@
   // Set the program counter value
   virtual void SetPC(uintptr_t new_pc) = 0;
 
+  // Gets the given GPRs address.
+  virtual uintptr_t* GetGPRAddress(uint32_t reg) = 0;
+
   // Read the given GPR
   virtual uintptr_t GetGPR(uint32_t reg) = 0;