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

common/hdr_util.h

@@ -12,14 +12,13 @@
 
 #include "mkvmuxer/mkvmuxer.h"
 
-namespace libwebm {
-
 namespace mkvparser {
 struct Colour;
 struct MasteringMetadata;
 struct PrimaryChromaticity;
 }  // namespace mkvparser
 
+namespace libwebm {
 // Utility types and functions for working with the Colour element and its
 // children. Copiers return true upon success. Presence functions return true
 // when the specified element is present.