Refactor implementation for String.prototype.replace.

src/jsregexp.cc

Index: src/jsregexp.cc
diff --git a/src/jsregexp.cc b/src/jsregexp.cc
index 8928d4e2b643cf1d4def309051ced0f74f598d49..aa75d2b7ee297e3b5e7dfa96be90656a74565ce2 100644
--- a/src/jsregexp.cc
+++ b/src/jsregexp.cc
@@ -687,6 +687,7 @@ Handle<JSArray> RegExpImpl::SetLastMatchInfo(Handle<JSArray> last_match_info,
                                              Handle<String> subject,
                                              int capture_count,
                                              int32_t* match) {
+  ASSERT(last_match_info->HasFastObjectElements());
   int capture_register_count = (capture_count + 1) * 2;
   last_match_info->EnsureSize(capture_register_count + kLastMatchOverhead);
   AssertNoAllocation no_gc;