Support string add in crankshaft:

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index f179865348fddcedd958f933770088f520426081..5df68020b8a22b2f040e07c42140b244333889cd 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1945,6 +1945,13 @@ LInstruction* LChunkBuilder::DoStoreNamedGeneric(HStoreNamedGeneric* instr) {
 }
 
 
+LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
+  LOperand* left = UseRegisterAtStart(instr->left());
+  LOperand* right = UseRegisterAtStart(instr->right());
+  return MarkAsCall(DefineFixed(new LStringAdd(left, right), r0), instr);
+}
+
+
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
   LOperand* string = UseRegister(instr->string());
   LOperand* index = UseRegisterOrConstant(instr->index());