Remove mock_ffmpeg and update media unittests.

media/base/media_posix.cc

Index: media/base/media_posix.cc
diff --git a/media/base/media_posix.cc b/media/base/media_posix.cc
index adad88bdc2e795392fac3f55952f7ceccfb8e8b3..2f794452b1f34f929cd154c658a6dd623cfad2fb 100644
--- a/media/base/media_posix.cc
+++ b/media/base/media_posix.cc
@@ -92,6 +92,13 @@ bool InitializeMediaLibrary(const FilePath& module_dir) {
   return g_media_library_is_initialized;
 }
 
+bool InitializeMediaLibraryForTesting() {
+  FilePath file_path;
+  if (!PathService::Get(base::DIR_SOURCE_ROOT, &file_path))
+    return false;
+  return InitializeMediaLibrary(file_path);
+}
+
 bool IsMediaLibraryInitialized() {
   return g_media_library_is_initialized;
 }