Add use_v4lplugin flag to enable v4l2 library

content/common/gpu/media/v4l2.sig

+// 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_open(const char *file, int oflag, ...);
+LIBV4L_PUBLIC int v4l2_close(int fd);
+LIBV4L_PUBLIC int v4l2_dup(int fd);
+LIBV4L_PUBLIC int v4l2_ioctl(int fd, unsigned long int request, ...);
+LIBV4L_PUBLIC ssize_t v4l2_read(int fd, void *buffer, size_t n);
+LIBV4L_PUBLIC ssize_t v4l2_write(int fd, const void *buffer, size_t n);
+LIBV4L_PUBLIC void *v4l2_mmap(void *start, size_t length, int prot, int flags, int fd, int64_t offset);
+LIBV4L_PUBLIC int v4l2_munmap(void *_start, size_t length);
+LIBV4L_PUBLIC int v4l2_fd_open(int fd, int v4l2_flags);