Add use_v4lplugin flag to enable v4l2 library

content/common/gpu/media/v4l2.sig

Index: content/common/gpu/media/v4l2.sig
diff --git a/content/common/gpu/media/v4l2.sig b/content/common/gpu/media/v4l2.sig
new file mode 100644
index 0000000000000000000000000000000000000000..4269fb48d592d5700119757cfcabb9d61c42b565
--- /dev/null
+++ b/content/common/gpu/media/v4l2.sig
@@ -0,0 +1,10 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+//------------------------------------------------
+// Functions from libv4l2 used in chromium code.
+//------------------------------------------------
+LIBV4L_PUBLIC int v4l2_close(int fd);
+LIBV4L_PUBLIC int v4l2_ioctl(int fd, unsigned long int request, ...);
+LIBV4L_PUBLIC int v4l2_fd_open(int fd, int v4l2_flags);