Use a JS markup serializer.

This makes MarkupAccumulator and all related code into dead code.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/771093002

engine/src/flutter/tests/parser/html-expected.txt

@@ -17,6 +17,7 @@ M <p></p>
 N 
 O <br></br>
 P 
+Q <template><div>contents</div></template>
 
 </isindex></keygen></body>
 </html>

engine/src/flutter/tests/parser/html.sky

@@ -19,5 +19,6 @@ M <p></p>
 N </p>
 O <br/>
 P </br>
+Q <template><div>contents</div></template>
 
 </body>