Fix test-hashing after recent changes to string hashing.

src/mips/deoptimizer-mips.cc

Index: src/mips/deoptimizer-mips.cc
===================================================================
--- src/mips/deoptimizer-mips.cc	(revision 10343)
+++ src/mips/deoptimizer-mips.cc	(working copy)
@@ -733,10 +733,7 @@
     }
   }
 
-  // Set up the roots register.
-  ExternalReference roots_array_start =
-      ExternalReference::roots_array_start(isolate);
-  __ li(roots, Operand(roots_array_start));
+  __ InitializeRootRegister();
 
   __ pop(at);  // Get continuation, leave pc on stack.
   __ pop(ra);