Mac: Avoid [AVFoundationBundle load] every time that IsAVFoundationSu…

…pported() is called

Change the result of AVFoundationGlue::IsAVFoundationSupported() to
be evaluated only once, and return the static bool every time.

The order of the flag evaluation changes subtly:
- First we check for OS version: there's no AVFoundation
in Mac OS X < 10.7.
- Second comes the check for command line flag for explicit
AVFoundation disabling (hence using QTKit).
- Third and final, the library tries to be loaded

BUG=288562, 342790

Review URL: https://codereview.chromium.org/185233003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254845 0039d316-1c4b-4281-b951-d872f2087c98

media/video/capture/mac/avfoundation_glue.mm

@@ -119,9 +119,12 @@
     LAZY_INSTANCE_INITIALIZER;
 
 bool AVFoundationGlue::IsAVFoundationSupported() {
-  const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
-  return (!cmd_line->HasSwitch(switches::kDisableAVFoundation) &&
-      base::mac::IsOSLionOrLater() && [AVFoundationBundle() load]);
+  // DeviceMonitorMac will initialize this static bool from the main UI thread
+  // once, during Chrome startup so this construction is thread safe.
+  static bool is_av_foundation_supported = base::mac::IsOSLionOrLater() &&
+      !CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kDisableAVFoundation) && [AVFoundationBundle() load];
+  return is_av_foundation_supported;
 }
 
 NSBundle const* AVFoundationGlue::AVFoundationBundle() {