Add kHeaderSize constant to SeqString.

src/mips/code-stubs-mips.cc

Index: src/mips/code-stubs-mips.cc
diff --git a/src/mips/code-stubs-mips.cc b/src/mips/code-stubs-mips.cc
index 28b8b367547b59144497359500fa98347121d9b1..f64973a6e95402428ccde461f7436a981bc329af 100644
--- a/src/mips/code-stubs-mips.cc
+++ b/src/mips/code-stubs-mips.cc
@@ -4721,8 +4721,7 @@ void RegExpExecStub::Generate(MacroAssembler* masm) {
 
   // For arguments 4 and 3 get string length, calculate start of string data
   // and calculate the shift of the index (0 for ASCII and 1 for two byte).
-  STATIC_ASSERT(SeqAsciiString::kHeaderSize == SeqTwoByteString::kHeaderSize);
-  __ Addu(t2, subject, Operand(SeqAsciiString::kHeaderSize - kHeapObjectTag));
+  __ Addu(t2, subject, Operand(SeqString::kHeaderSize - kHeapObjectTag));
   __ Xor(a3, a3, Operand(1));  // 1 for 2-byte str, 0 for 1-byte.
   // Load the length from the original subject string from the previous stack
   // frame. Therefore we have to use fp, which points exactly to two pointer