Support string add in crankshaft:

src/ic.h

Index: src/ic.h
diff --git a/src/ic.h b/src/ic.h
index 911cbd83575acb9107ff1384d54a4be410872df4..342447a68c6f8c04ba0110817b01f7534e18c389 100644
--- a/src/ic.h
+++ b/src/ic.h
@@ -586,6 +586,7 @@ class TRBinaryOpIC: public IC {
     INT32,
     HEAP_NUMBER,
     ODDBALL,
+    BOTH_STRING,  // Only used for addition operation.
     STRING,  // Only used for addition operation.  At least one string operand.
     GENERIC
   };