Change static_cast to To<> in ReadableStreamDefaultController

It is generally safer/preferred to use blink::To instead of static_cast
to a derived class from a base class, as it has a security check that
the object is actually of the derived class type. Hence, this CL
changes the static_cast to use To<ReadableStreamDefaultReader> instead.

Bug: 614302
Change-Id: I93d9fe8a833f833c3ba92e84dd02ed0ad3356e3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577390Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Nidhi Jaju <nidhijaju@google.com>
Cr-Commit-Position: refs/heads/master@{#834595}

third_party/blink/renderer/core/streams/readable_stream_default_controller.cc

@@ -316,10 +316,10 @@ StreamPromiseResolver* ReadableStreamDefaultController::PullSteps(
     //    stream.[[reader]].[[forAuthorCode]]).
     ReadableStreamGenericReader* reader = stream->reader_;
     return StreamPromiseResolver::CreateResolved(
-        script_state, ReadableStream::CreateReadResult(
-                          script_state, chunk, false,
-                          static_cast<ReadableStreamDefaultReader*>(reader)
-                              ->for_author_code_));
+        script_state,
+        ReadableStream::CreateReadResult(
+            script_state, chunk, false,
+            To<ReadableStreamDefaultReader>(reader)->for_author_code_));
   }
 
   // 3. Let pendingPromise be ! ReadableStreamAddReadRequest(stream).