[builtins] don't pre-flatten strings for JSStringIterator

Seems to be pretty perf-neutral (for the benchmarks we have in the tree) against
the pre-flattened version.

BUG=v8:5388
R=bmeurer@chromium.org, adamk@chromium.org, littledan@chromium.org

Review-Url: https://codereview.chromium.org/2382013002
Cr-Commit-Position: refs/heads/master@{#39907}

src/objects-debug.cc

@@ -785,7 +785,7 @@ void JSWeakMap::JSWeakMapVerify() {
 void JSStringIterator::JSStringIteratorVerify() {
   CHECK(IsJSStringIterator());
   JSObjectVerify();
-  CHECK(string()->IsSeqString() || string()->IsExternalString());
+  CHECK(string()->IsString());
 
   CHECK_GE(index(), 0);
   CHECK_LE(index(), String::kMaxLength);