Fix zero hash handling on ARM.

src/arm/code-stubs-arm.cc

Index: src/arm/code-stubs-arm.cc
===================================================================
--- src/arm/code-stubs-arm.cc	(revision 10360)
+++ src/arm/code-stubs-arm.cc	(working copy)
@@ -5759,13 +5759,12 @@
   // hash ^= hash >> 11;
   __ eor(hash, hash, Operand(hash, LSR, 11));
   // hash += hash << 15;
-  __ add(hash, hash, Operand(hash, LSL, 15), SetCC);
+  __ add(hash, hash, Operand(hash, LSL, 15));
 
-  uint32_t kHashShiftCutOffMask = (1 << (32 - String::kHashShift)) - 1;
-  __ and_(hash, hash, Operand(kHashShiftCutOffMask));
+  __ and_(hash, hash, Operand(String::kHashBitMask), SetCC);
 
   // if (hash == 0) hash = 27;
-  __ mov(hash, Operand(27), LeaveCC, eq);
+  __ mov(hash, Operand(StringHasher::kZeroHash), LeaveCC, eq);
 }