Add Support for Audio MIME Types

The EME spec technically requires CDMs to treat audio/mp4 and
video/mp4 equivalently, as well as audio/webm and video/webm. We had
only been accepting video/mp4 and video/webm up until now.

This change also centralizes handling of init data types in the shared
CDM code instead of having it spread across multiple places in the
codebase.

(This is a merge of https://widevine-internal-review.googlesource.com/9532/
from the Widevine CDM repo.)

Bug: 13564917
Change-Id: Ib8bdfb2b003ffb00e8f0559561335abb3c5778b0

libwvdrmengine/cdm/include/wv_content_decryption_module.h

@@ -18,6 +18,9 @@ class WvContentDecryptionModule {
   WvContentDecryptionModule();
   virtual ~WvContentDecryptionModule();
 
+  // Static methods
+  static bool SupportsInitDataType(const std::string& type);
+
   // Session related methods
   virtual CdmResponseType OpenSession(
       const CdmKeySystem& key_system,
@@ -28,7 +31,7 @@ class WvContentDecryptionModule {
   // Construct a valid license request.
   virtual CdmResponseType GenerateKeyRequest(const CdmSessionId& session_id,
                                              const CdmKeySetId& key_set_id,
-                                             const std::string& mime_type,
+                                             const std::string& init_data_type,
                                              const CdmInitData& init_data,
                                              const CdmLicenseType license_type,
                                              CdmAppParameterMap& app_parameters,