Use a UI thread for capturing on macOS.

https://chromium-review.googlesource.com/c/chromium/src/+/681850 changed
the default run loop on Mac, which broke our capturer (it started
returning zero-size frames for all but the initial capture). I'm not
sure what our dependency on MessagePumpCFRunLoop is, but switching to a
UI thread means that the capturer has access to one again.

Change-Id: I9ac8ac19f325f88f9e758613f50ac2a2a00dc01d
Bug: 827300
Reviewed-on: https://chromium-review.googlesource.com/996296Reviewed-by: Joe Downing <joedow@chromium.org>
Commit-Queue: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548278}

remoting/host/chromoting_host_context.cc

@@ -130,7 +130,13 @@ std::unique_ptr<ChromotingHostContext> ChromotingHostContext::Create(
       AutoThread::CreateWithType("ChromotingInputThread", ui_task_runner,
                                  base::MessageLoop::TYPE_IO),
       network_task_runner,
+#if defined(OS_MACOSX)
+      // Mac requires a UI thread for the capturer.
+      AutoThread::CreateWithType("ChromotingCaptureThread", ui_task_runner,
+                                 base::MessageLoop::TYPE_UI),
+#else
       AutoThread::Create("ChromotingCaptureThread", ui_task_runner),
+#endif
       AutoThread::Create("ChromotingEncodeThread", ui_task_runner),
       base::MakeRefCounted<URLRequestContextGetter>(network_task_runner,
                                                     file_task_runner),