Merge V8 at r10375: Roll to 3.6.6.17

Bug: 5688872
Change-Id: I558f9b89a15d2dcf1b62dcf9f297d4d42ca5830c
diff --git a/src/mips/macro-assembler-mips.h b/src/mips/macro-assembler-mips.h
index 5dd012e..ce19a4e 100644
--- a/src/mips/macro-assembler-mips.h
+++ b/src/mips/macro-assembler-mips.h
@@ -671,6 +671,11 @@
   void DebugBreak();
 #endif
 
+  void InitializeRootRegister() {
+    ExternalReference roots_address =
+        ExternalReference::roots_address(isolate());
+    li(kRootRegister, Operand(roots_address));
+  }
 
   // -------------------------------------------------------------------------
   // Exception handling.