Add WARN_UNUSED_RESULT to crypto/hmac.h

crypto/hmac_mac.cc

Index: crypto/hmac_mac.cc
diff --git a/crypto/hmac_mac.cc b/crypto/hmac_mac.cc
index f9a00cda8adaab7da1a65f24187b5b6f0a70e812..36caa0df8ebf71c50c51f0285469857422294960 100644
--- a/crypto/hmac_mac.cc
+++ b/crypto/hmac_mac.cc
@@ -42,6 +42,12 @@ HMAC::~HMAC() {
 bool HMAC::Sign(const base::StringPiece& data,
                 unsigned char* digest,
                 int digest_length) const {
+  if (plat_->key_.empty()) {
+    // Init has not been called or has failed.
+    NOTREACHED();
+    return false;
+  }
+
   CCHmacAlgorithm algorithm;
   int algorithm_digest_length;
   switch (hash_alg_) {