Use ScopedTaskEnvironment instead of MessageLoop in /ui/gfx/range/mojo

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

Change-Id: I4f758e010b973362f1c64f8fe6b821ec6f90e760
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643867Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#666631}

ui/gfx/range/mojo/BUILD.gn

@@ -32,6 +32,7 @@ source_set("unit_test") {
   deps = [
     ":test_interfaces",
     "//base",
+    "//base/test:test_support",
     "//mojo/public/cpp/bindings",
     "//testing/gtest",
     "//ui/gfx/range",

ui/gfx/range/mojo/range_struct_traits_unittest.cc

@@ -4,7 +4,7 @@
 
 #include <utility>
 
-#include "base/message_loop/message_loop.h"
+#include "base/test/scoped_task_environment.h"
 #include "mojo/public/cpp/bindings/binding_set.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/range/mojo/range_traits_test_service.mojom.h"
@@ -35,7 +35,7 @@ class RangeStructTraitsTest : public testing::Test,
     std::move(callback).Run(p);
   }
 
-  base::MessageLoop loop_;
+  base::test::ScopedTaskEnvironment scoped_task_environment_;
   mojo::BindingSet<RangeTraitsTestService> traits_test_bindings_;
 
   DISALLOW_COPY_AND_ASSIGN(RangeStructTraitsTest);