Add use_v4lplugin flag to enable v4l2 library

content/common/gpu/media/generic_v4l2_video_device.h

Index: content/common/gpu/media/generic_v4l2_video_device.h
diff --git a/content/common/gpu/media/generic_v4l2_video_device.h b/content/common/gpu/media/generic_v4l2_video_device.h
index e7560d177a05f51477eac05757c3cac179685abe..d098fe1c030ae9b035313d8a31302799bbaa6ff2 100644
--- a/content/common/gpu/media/generic_v4l2_video_device.h
+++ b/content/common/gpu/media/generic_v4l2_video_device.h
@@ -52,6 +52,10 @@ class GenericV4L2Device : public V4L2Device {
   int device_poll_interrupt_fd_;
 
   DISALLOW_COPY_AND_ASSIGN(GenericV4L2Device);
+
+  // Lazily initialize static data after sandbox is enabled.  Return false on
+  // init failure.
+  static bool PostSandboxInitialization();
 };
 }  //  namespace content