Unify the error checking of crypto::Encryptor and add WARN_UNUSED_RESULT to prevent misuse.

crypto/encryptor.cc

Index: crypto/encryptor.cc
diff --git a/crypto/encryptor.cc b/crypto/encryptor.cc
index 763dc2c1cb54764ab2428054f9585616f6decdde..0aa706c9b4c14d017295f90fbc00957df99e9e1e 100644
--- a/crypto/encryptor.cc
+++ b/crypto/encryptor.cc
@@ -88,13 +88,13 @@ bool Encryptor::SetCounter(const base::StringPiece& counter) {
 bool Encryptor::GenerateCounterMask(size_t plaintext_len,
                                     uint8* mask,
                                     size_t* mask_len) {
-  DCHECK_EQ(CTR, mode_);
-  CHECK(mask);
-  CHECK(mask_len);
+  if (!mask || !mask_len || mode_ != CTR)
+    return false;
 
   const size_t kBlockLength = counter_->GetLengthInBytes();
   size_t blocks = (plaintext_len + kBlockLength - 1) / kBlockLength;
-  CHECK(blocks);
+  if (blocks == 0)
+    return false;
 
   *mask_len = blocks * kBlockLength;