Use ScopedTaskEnvironment instead of MessageLoop in /extensions/renderer

MessageLoop will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you belive your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=lazyboy@chromium.org

Change-Id: I9696c08d614a0842e494e7fd40566d72235d85fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649509
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#667175}

extensions/renderer/script_context_set_unittest.cc

@@ -4,8 +4,8 @@
 
 #include <vector>
 
-#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "extensions/common/extension.h"
 #include "extensions/common/extension_set.h"
 #include "extensions/common/features/feature.h"
@@ -21,7 +21,7 @@
 namespace extensions {
 
 TEST(ScriptContextSetTest, Lifecycle) {
-  base::MessageLoop loop;
+  base::test::ScopedTaskEnvironment scoped_task_environment;
   ScopedWebFrame web_frame;
   // Used by ScriptContextSet::Register().
   TestExtensionsRendererClient extensions_renderer_client;