Move EmptyString, kWhitespace and the BOM to base.

media/base/android/media_drm_bridge.cc

Index: media/base/android/media_drm_bridge.cc
diff --git a/media/base/android/media_drm_bridge.cc b/media/base/android/media_drm_bridge.cc
index 8b5d59e4136b3badb15d61afba8b365e269e77eb..6a72e179965aafee86303617baed65aaf476bd01 100644
--- a/media/base/android/media_drm_bridge.cc
+++ b/media/base/android/media_drm_bridge.cc
@@ -392,7 +392,7 @@ uint32_t MediaDrmBridge::DetermineReferenceId(const std::string& session_id) {
 const std::string& MediaDrmBridge::LookupSessionId(uint32 reference_id) {
   // Session may not exist if error happens during GenerateKeyRequest().
   SessionMap::iterator it = session_map_.find(reference_id);
-  return (it != session_map_.end()) ? it->second : EmptyString();
+  return (it != session_map_.end()) ? it->second : base::EmptyString();
 }
 
 }  // namespace media