Don't initialize decryptor on CDM creation failure

PpapiDecryptor::Create() must not proceed with decryptor initialization
if the CDM wrapper could not be created.
PpapiDecryptor::PpapiDecryptor() asserts the CDM wrapper is valid and
InitializeCdm() uses it unconditionally.

BUG=488363
TEST=EME tests pass

Review URL: https://codereview.chromium.org/1128693004

Cr-Commit-Position: refs/heads/master@{#330150}

content/renderer/media/crypto/ppapi_decryptor.cc

@@ -44,6 +44,7 @@ void PpapiDecryptor::Create(
     base::MessageLoopProxy::current()->PostTask(
         FROM_HERE, base::Bind(cdm_created_cb, nullptr,
                               "Plugin instance creation failed."));
+    return;
   }
 
   scoped_ptr<PpapiDecryptor> ppapi_decryptor(