Restore original namespaces for mkvmuxer and mkvparser.

Wrapping mkvmuxer and mkvparser in the libwebm namespace is no
longer necessary now that the tree reorganization is complete.
Put mkvmuxer and mkvparser namespaces back in the global
namespace to avoid unnecessary churn in downstream code.

Change-Id: I13a4fe0143d20bb2bb6038078c68636ff2af0c29

testing/test_util.h

@@ -13,7 +13,6 @@
 #include <cstddef>
 #include <string>
 
-namespace libwebm {
 namespace test {
 
 // constants for muxer and parser tests
@@ -54,6 +53,5 @@ std::string GetTestFilePath(const std::string& name);
 bool CompareFiles(const std::string& file1, const std::string& file2);
 
 }  // namespace test
-}  // namespace libwebm
 
 #endif  // LIBWEBM_TESTING_TEST_UTIL_H_