Revert "Do not use soon deprecated v8::ReturnValue::Set."

This reverts commit 7ef680c7.

Reason for revert: Performance regression crbug.com/802401

Original change's description:
> Do not use soon deprecated v8::ReturnValue::Set.
> 
> Bug: v8:7289
> Change-Id: I4b7bc2640d8103b925a69e95b9687011084e0f46
> Reviewed-on: https://chromium-review.googlesource.com/863742
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#528947}

TBR=haraken@chromium.org,yangguo@chromium.org,mlippautz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7289
Change-Id: I5b8720c1eb460ffb2bd6d8774c0c3e7957d48ce5
Reviewed-on: https://chromium-review.googlesource.com/893358Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533612}

third_party/WebKit/Source/platform/bindings/ScriptWrappable.h

@@ -141,8 +141,7 @@ class PLATFORM_EXPORT ScriptWrappable
   }
 
   bool SetReturnValue(v8::ReturnValue<v8::Value> return_value) {
-    v8::Isolate* isolate = return_value.GetIsolate();
-    return_value.Set(main_world_wrapper_.Get().Get(isolate));
+    return_value.Set(main_world_wrapper_.Get());
     return ContainsWrapper();
   }