[MessageLoop]: Replace RunLoop-related ScopedNestableTaskAllower in /content/renderer.

This CL replaces ScopedNestableTaskAllower with kNestableTasksAllowed passed
as argument to the RunLoop involved if any.
Only native nested loops should use ScopedNestableTaskAllower (soon to be deprecated).
For more details, see:
https://cs.chromium.org/chromium/src/base/message_loop/message_loop_current.h?sq=package:chromium&dr=CSs&g=0&l=159

A small step towards deleting MessageLoop.

This CL was generated with git cl split.
Refer to the top-level CL if necessary :
https://chromium-review.googlesource.com/c/chromium/src/+/1635819

Please CQ if LGTY!

This CL was uploaded by git cl split.

R=dcheng@chromium.org

Bug: 766678
Change-Id: I59de40f915421970b5da834ac2240c733581d8f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636057
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667325}

content/renderer/dom_serializer_browsertest.cc

@@ -134,7 +134,6 @@ class MAYBE_DomSerializerTests : public ContentBrowserTest,
         contents, base_url,
         encoding_info.IsEmpty() ? "UTF-8" : encoding_info.Utf8(), GURL(),
         false /* replace_current_item */);
-    base::MessageLoopCurrent::ScopedNestableTaskAllower allow;
     waiter.Wait();
   }