Make ReadableStream extendable

IDL defined ReadableStream was not extendable because it didn't use
provided |info.Holder()| as its wrapper. Instead, it creates a new
wrapper in Create functions.

This CL solves the issue by making the constructor "custom". The custom
constructor creates a blink::ReadableStream first, then associates it
to the wrapper, and then calls the appropriate Init function.

Bug: 906476
Change-Id: I91a7fec1cf4ee5e08704a944b0af852584d1679e
Reviewed-on: https://chromium-review.googlesource.com/c/1347643
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610391}

streams/readable-streams/general.js

@@ -774,6 +774,13 @@ test(() => {
   });
 }, 'ReadableStream: desiredSize when errored');
 
+test(() => {
+   class Extended extends ReadableStream {
+     newMethod() { return 'foo' };
+   };
+   assert_equals((new Extended()).newMethod(), 'foo');
+}, 'ReadableStream: ReadableStream is extendable');
+
 test(() => {
 
   let startCalled = false;