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..7be3255f2af2c30ffd1fc7fbc81c093d17fb80b7 100644
--- a/media/base/media_posix.cc
+++ b/media/base/media_posix.cc
@@ -92,6 +92,12 @@ bool InitializeMediaLibrary(const FilePath& module_dir) {
   return g_media_library_is_initialized;
 }
 
+void InitializeMediaLibraryForTesting() {
+  FilePath file_path;
+  CHECK(PathService::Get(base::DIR_EXE, &file_path));
+  CHECK(InitializeMediaLibrary(file_path));
+}
+
 bool IsMediaLibraryInitialized() {
   return g_media_library_is_initialized;
 }