Optimize flattening of cons strings with equal left and right-side.

This significantly speeds up flattening repeat strings (e.g., '.'.repeat(1000)).

BUG=

Review URL: https://codereview.chromium.org/1798073002

Cr-Commit-Position: refs/heads/master@{#34751}

src/objects.cc

@@ -11539,6 +11539,10 @@ void String::WriteToFlat(String* src,
           // Right hand side is longer.  Recurse over left.
           if (from < boundary) {
             WriteToFlat(first, sink, from, boundary);
+            if (from == 0 && cons_string->second() == first) {
+              CopyChars(sink + boundary, sink, boundary);
+              return;
+            }
             sink += boundary - from;
             from = 0;
           } else {