Expose Controller() in UnderlyingSinkBase

In the JavaScript API for underlying sinks, they usually capture the
controller if they need it in the start() method. However, the C++
UnderlyingSinkBase needs to store the controller internally for its own
use. It's wasteful for subclasses to take their own copy. Instead expose
it to subclasses directly via a protected getter.

This getter will be used by the implementation of WebSocketStream. See
the design doc at
https://docs.google.com/document/d/1XuxEshh5VYBYm1qRVKordTamCOsR-uGQBCYFcHXP4L0/edit.

Also fix a bug in the generic version of
WritableStreamDefaultControllerInterface::Error().

BUG=983030

Change-Id: I5339f9117f52b78947592b417cb595b2fa496c3f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1734738Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684284}

third_party/blink/renderer/core/streams/underlying_sink_base.h

@@ -50,6 +50,11 @@ class CORE_EXPORT UnderlyingSinkBase : public ScriptWrappable {
     ScriptWrappable::Trace(visitor);
   }
 
+ protected:
+  WritableStreamDefaultControllerInterface* Controller() const {
+    return controller_;
+  }
+
  private:
   Member<WritableStreamDefaultControllerInterface> controller_;
 };

third_party/blink/renderer/core/streams/writable_stream_default_controller_interface.h

@@ -33,7 +33,7 @@ class CORE_EXPORT WritableStreamDefaultControllerInterface
   // Helper method
   template <typename ErrorType>
   void Error(ScriptState* script_state, ErrorType error) {
-    Error(script_state, ToV8(error, script_state).ToV8Value());
+    Error(script_state, ToV8(error, script_state));
   }
 
   virtual void Trace(Visitor*) {}