translate: Use a per-frame task runner at TranslateHelper

V8 must not be executed on the default task runner. This CL replaces
that with a per-frame task runner.

Bug: 870606
Change-Id: I9c3844692235baad411a83ed50a7c6402e0a3b8f
Reviewed-on: https://chromium-review.googlesource.com/c/1312444Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610007}

components/translate/content/renderer/translate_helper.cc

@@ -134,7 +134,9 @@ void TranslateHelper::PageCaptured(const base::string16& contents) {
   // captured, it should be treated as a new page to do translation.
   ResetPage();
   mojom::PagePtr page;
-  binding_.Bind(mojo::MakeRequest(&page));
+  binding_.Bind(
+      mojo::MakeRequest(&page),
+      main_frame->GetTaskRunner(blink::TaskType::kInternalTranslation));
   GetTranslateHandler()->RegisterPage(
       std::move(page), details, !details.has_notranslate && !language.empty());
 }