Switch to using a UI type MessagePump for macOS media_unitests.

For some reason the MessagePumpDefault isn't scheduled with enough
precision to execute 20ms interval callbacks, so our fake audio path
ends up lagging further and further behind and slowing down tests.

BUG=1003761,1014646
TEST=osxbeta-rel trybot passes.
R=tmathmeyer

Change-Id: Icc0fa9ef9ff698d866ff77aab86206be2f16a23d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906120Reviewed-by: Ted Meyer <tmathmeyer@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714041}

media/test/pipeline_integration_test_base.cc

@@ -13,6 +13,7 @@
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/time/time.h"
+#include "build/build_config.h"
 #include "media/base/media_log.h"
 #include "media/base/media_switches.h"
 #include "media/base/media_tracks.h"
@@ -125,7 +126,14 @@ class RendererFactoryImpl final : public PipelineTestRendererFactory {
 };
 
 PipelineIntegrationTestBase::PipelineIntegrationTestBase()
-    : hashing_enabled_(false),
+    :
+// Use a UI type message loop on macOS, because it doesn't seem to schedule
+// callbacks with enough precision to drive our fake audio output. See
+// https://crbug.com/1014646 for more details.
+#if defined(OS_MACOSX)
+      task_environment_(base::test::TaskEnvironment::MainThreadType::UI),
+#endif
+      hashing_enabled_(false),
       clockless_playback_(false),
       webaudio_attached_(false),
       mono_output_(false),