[Video Capture] Add feature switch for DeviceMonitorMac

On Mac it can happen that certain cameras stop working system-wide after Chrome 
has been running for a while (potentially days) and the computer has gone 
through sleep/wakeup cycles.

This could potentially be related to DeviceMonitorMac interacting with 
VDCAssistant either at a bad time or in a bad way. To be able to test this 
theory, this CL adds a feature switch that allows disabling the use of 
DeviceMonitorMac.

Bug: 699446
Test: Run chromium on Mac with --disable-features=DeviceMonitorMac
Change-Id: Id2a06c75bca7ec8e2db37b4b9647fe508edb0327
Reviewed-on: https://chromium-review.googlesource.com/558199Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Emircan Uysaler <emircan@chromium.org>
Commit-Queue: Christian Fremerey <chfremer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486933}

content/browser/renderer_host/media/media_devices_manager.cc

@@ -20,6 +20,7 @@
 #include "content/browser/renderer_host/media/media_stream_manager.h"
 #include "content/browser/renderer_host/media/video_capture_manager.h"
 #include "content/public/browser/browser_thread.h"
+#include "content/public/common/content_features.h"
 #include "media/audio/audio_device_description.h"
 #include "media/audio/audio_system.h"
 #include "media/base/media_switches.h"
@@ -232,6 +233,11 @@ void MediaDevicesManager::StartMonitoring() {
   if (!base::SystemMonitor::Get())
     return;
 
+#if defined(OS_MACOSX)
+  if (!base::FeatureList::IsEnabled(features::kDeviceMonitorMac))
+    return;
+#endif
+
   monitoring_started_ = true;
   base::SystemMonitor::Get()->AddDevicesChangedObserver(this);
 

content/public/common/content_features.cc

@@ -411,6 +411,10 @@ const base::Feature kWinSboxDisableExtensionPoints{
 #endif
 
 #if defined(OS_MACOSX)
+// Enables caching of media devices for the purpose of enumerating them.
+const base::Feature kDeviceMonitorMac{"DeviceMonitorMac",
+                                      base::FEATURE_ENABLED_BY_DEFAULT};
+
 // The V2 sandbox on MacOS removes the unsandboed warmup phase and sandboxes the
 // entire life of the process.
 const base::Feature kMacV2Sandbox{"MacV2Sandbox",

content/public/common/content_features.h

@@ -109,6 +109,7 @@ CONTENT_EXPORT extern const base::Feature kWinSboxDisableExtensionPoints;
 #endif  // defined(OS_WIN)
 
 #if defined(OS_MACOSX)
+CONTENT_EXPORT extern const base::Feature kDeviceMonitorMac;
 CONTENT_EXPORT extern const base::Feature kMacV2Sandbox;
 #endif  // defined(OS_MACOSX)