NewProvisioningSession expects pkcs8 private key and SHA race fix

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Fix SHA hashing to remove race condition. This change
fixes the implementation by passing in the digest buffer.

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The input to ProvisioningEngine::NewProvisioningSession should be
pkcs8 private key instead of pkcs1 private key

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=151273394

Change-Id: Ibcdff7757b2ac2878ee8b1b88365083964bfa10a

common/random_util.cc

@@ -19,4 +19,9 @@ bool RandomBytes(size_t num_bytes, std::string* output) {
   return RAND_bytes(reinterpret_cast<uint8_t*>(&(*output)[0]), num_bytes);
 }
 
+std::string Random16Bytes() {
+  std::string output;
+  CHECK(RandomBytes(16u, &output));
+  return output;
+}
 }  // namespace widevine