Mac: Add flag to force QTKit in otherwise AVFoundation-enabled builds/configs.

History: AVFoundation and QTKit are the two options for audio/video 
monitoring and video capture in Mac. AVFoundation is currently enabled
in Mac OS X >= 10.7, and only via command line flag, which is currently
enabled in developer and Canary builds. 

This CL adds a --force-qtkit flag that suppresses AVFoundation usage
for those versions and channels that otherwise would use it.

Please note the naming: --force-qtkit vs. --enable-avfoundation

BUG=288562, 369024, 357250

Review URL: https://codereview.chromium.org/260873011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268065 0039d316-1c4b-4281-b951-d872f2087c98

media/base/media_switches.cc

@@ -40,6 +40,11 @@ const char kAlsaOutputDevice[] = "alsa-output-device";
 // for experimentation purposes, in particular library load time issue, the
 // usage of this library can be enabled by using this flag.
 const char kEnableAVFoundation[] = "enable-avfoundation";
+// QTKit is the media capture API predecessor to AVFoundation, available up and
+// until Mac OS X 10.9 (despite being deprecated in this last one). This flag
+// is used for troubleshooting and testing, and forces QTKit in builds and
+// configurations where AVFoundation would be used otherwise.
+const char kForceQTKit[] = "force-qtkit";
 #endif
 
 #if defined(OS_WIN)

media/base/media_switches.h

@@ -30,6 +30,7 @@ MEDIA_EXPORT extern const char kAlsaOutputDevice[];
 
 #if defined(OS_MACOSX)
 MEDIA_EXPORT extern const char kEnableAVFoundation[];
+MEDIA_EXPORT extern const char kForceQTKit[];
 #endif
 
 #if defined(OS_WIN)

media/video/capture/mac/avfoundation_glue.mm

@@ -108,11 +108,13 @@ static base::LazyInstance<AVFoundationInternal> g_avfoundation_handle =
 bool AVFoundationGlue::IsAVFoundationSupported() {
   // DeviceMonitorMac will initialize this static bool from the main UI thread
   // once, during Chrome startup so this construction is thread safe.
+  // Use AVFoundation if possible, enabled, and QTKit is not explicitly forced.
+  static CommandLine* command_line = CommandLine::ForCurrentProcess();
   static bool is_av_foundation_supported = base::mac::IsOSLionOrLater() &&
-      (CommandLine::ForCurrentProcess()->HasSwitch(
-          switches::kEnableAVFoundation) ||
-          base::FieldTrialList::FindFullName("AVFoundationMacVideoCapture")
-              == "Enabled") && [AVFoundationBundle() load];
+      ((command_line->HasSwitch(switches::kEnableAVFoundation) &&
+      !command_line->HasSwitch(switches::kForceQTKit)) ||
+      base::FieldTrialList::FindFullName("AVFoundationMacVideoCapture")
+          == "Enabled") && [AVFoundationBundle() load];
   return is_av_foundation_supported;
 }