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

crypto/encryptor_unittest.cc

Index: crypto/encryptor_unittest.cc
diff --git a/crypto/encryptor_unittest.cc b/crypto/encryptor_unittest.cc
index 7f2caf9f7e2374720fb67e5d8478a9ed8a0f9282..4f470d4a56b97e85d00dd29558885031cd8e63ae 100644
--- a/crypto/encryptor_unittest.cc
+++ b/crypto/encryptor_unittest.cc
@@ -78,7 +78,7 @@ TEST(EncryptorTest, CTRCounter) {
   // Increment 10 times.
   crypto::Encryptor::Counter counter1(std::string(kTest1, kCounterSize));
   for (int i = 0; i < 10; ++i)
-    counter1.Increment();
+    EXPECT_TRUE(counter1.Increment());
   counter1.Write(buf);
   EXPECT_EQ(0, memcmp(buf, kTest1, 15));
   EXPECT_TRUE(buf[15] == 10);
@@ -88,7 +88,7 @@ TEST(EncryptorTest, CTRCounter) {
                          0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
   const char kExpect2[] = {0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0};
   crypto::Encryptor::Counter counter2(std::string(kTest2, kCounterSize));
-  counter2.Increment();
+  EXPECT_TRUE(counter2.Increment());
   counter2.Write(buf);
   EXPECT_EQ(0, memcmp(buf, kExpect2, kCounterSize));
 
@@ -96,7 +96,7 @@ TEST(EncryptorTest, CTRCounter) {
                          0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
   const char kExpect3[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
   crypto::Encryptor::Counter counter3(std::string(kTest3, kCounterSize));
-  counter3.Increment();
+  EXPECT_TRUE(counter3.Increment());
   counter3.Write(buf);
   EXPECT_EQ(0, memcmp(buf, kExpect3, kCounterSize));
 }