Enable snapshots on MIPS. This is based on

src/mips/full-codegen-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2595 matching lines @@
   // Calculate location of the first key name.
   __ Addu(t0,
           t0,
           Operand(FixedArray::kHeaderSize - kHeapObjectTag +
                   DescriptorArray::kFirstIndex * kPointerSize));
   // Loop through all the keys in the descriptor array. If one of these is the
   // symbol valueOf the result is false.
   Label entry, loop;
   // The use of t2 to store the valueOf symbol asumes that it is not otherwise
   // used in the loop below.
-  __ li(t2, Operand(FACTORY->value_of_symbol()));
+  __ LoadRoot(t2, Heap::kvalue_of_symbolRootIndex);
   __ jmp(&entry);
   __ bind(&loop);
   __ lw(a3, MemOperand(t0, 0));
   __ Branch(if_false, eq, a3, Operand(t2));
   __ Addu(t0, t0, Operand(kPointerSize));
   __ bind(&entry);
   __ Branch(&loop, ne, t0, Operand(a2));
 
   // If a valueOf property is not found on the object check that it's
   // prototype is the un-modified String prototype. If not result is false.
@@ skipping 1876 matching lines @@
   *context_length = 0;
   return previous_;
 }
 
 
 #undef __
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_MIPS