Use a message loop of type UI on the main thread of video-capture service

AVFoundation calls made by the video-capture service on its main thread
require a CFRunLoop which is provided by a UI message loop.
This fixes a number of regressions related to detection of plugged/unplugged
devices that appeared when the default message loops on Mac were changed from
CFRunLoop to Default.

Bug: 834581
Change-Id: If565f400e52407c2f71e32f93e9472556051bad1
Reviewed-on: https://chromium-review.googlesource.com/1049931Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557202}

chrome/browser/BUILD.gn

@@ -3047,6 +3047,7 @@ jumbo_split_static_library("browser") {
       "//chrome/app_shim",
       "//chrome/browser/apps/app_shim",
       "//chrome/browser/ui/cocoa/notifications:common",
+      "//services/video_capture/public/mojom:constants",
       "//third_party/crashpad/crashpad/client:client",
       "//third_party/google_toolbox_for_mac",
       "//third_party/mozilla",

chrome/browser/DEPS

@@ -78,6 +78,7 @@ include_rules = [
   "+services/ui/common",
   "+services/ui/public",
   "+services/ui/service.h",
+  "+services/video_capture/public",
   "+services/viz/privileged",
   "+skia/ext",
   "+third_party/boringssl/src/include",

chrome/browser/chrome_content_browser_client.cc

@@ -276,6 +276,7 @@
 #include "sandbox/win/src/sandbox_policy.h"
 #elif defined(OS_MACOSX)
 #include "chrome/browser/chrome_browser_main_mac.h"
+#include "services/video_capture/public/mojom/constants.mojom.h"
 #elif defined(OS_CHROMEOS)
 #include "ash/public/interfaces/constants.mojom.h"
 #include "chrome/browser/ash_service_registry.h"
@@ -2096,6 +2097,13 @@ void ChromeContentBrowserClient::AdjustUtilityServiceProcessCommandLine(
       command_line->AppendSwitchNative(sw.first, sw.second);
   }
 #endif  // BUILDFLAG(ENABLE_MUS)
+
+#if defined(OS_MACOSX)
+  // On Mac, the video-capture service requires a CFRunLoop, provided by a UI
+  // message loop, to run AVFoundation code. See https://crbug.com/834581
+  if (identity.name() == video_capture::mojom::kServiceName)
+    command_line->AppendSwitch(switches::kMessageLoopTypeUi);
+#endif
 }
 
 std::string ChromeContentBrowserClient::GetApplicationLocale() {