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/mips/context_mips.h b/runtime/arch/mips/context_mips.h
index 4145cd3..d5f27ae 100644
--- a/runtime/arch/mips/context_mips.h
+++ b/runtime/arch/mips/context_mips.h
@@ -43,6 +43,11 @@
     SetGPR(RA, new_pc);
   }
 
+  virtual uintptr_t* GetGPRAddress(uint32_t reg) {
+    DCHECK_LT(reg, static_cast<uint32_t>(kNumberOfCoreRegisters));
+    return gprs_[reg];
+  }
+
   virtual uintptr_t GetGPR(uint32_t reg) {
     CHECK_LT(reg, static_cast<uint32_t>(kNumberOfCoreRegisters));
     return *gprs_[reg];