Use Jsmn to parse json data

[ Merge of http://go/wvgerrit/16628 ]

Jsmn will replace a local method that parsed json init data.
Added a fix to include all key Ids in the WidevineCencHeader rather than
just the first. Also modified the content_id to reflect that it is a
base64 encoded value.

b/20630275

Change-Id: I7080c8cea21be4dea09a4905a96b4cc03e584c1d

libwvdrmengine/cdm/core/include/initialization_data.h

@@ -47,8 +47,6 @@ class InitializationData {
                                   std::vector<uint8_t>* value);
   static bool ExtractAttribute(const std::string& attribute_list,
                                const std::string& key, std::string* value);
-  static bool ExtractJsonValue(const std::string& json,
-                               const std::string& key, std::string* value);
 
   static std::vector<std::string> ExtractKeyFormatVersions(
       const std::string& key_format_versions);