Add Comment to DecryptWithNearWrap

Merge from widevine repo of http://go/wvgerrit/14933

There was some confusion what the test DecryptWithNearWrap is
testing.  This CL adds some expanatory comments.

Change-Id: I9228830d81c089f80e0878f647e7e94c3e49896a

libwvdrmengine/oemcrypto/test/oemcrypto_test.cpp

@@ -2689,6 +2689,15 @@ TEST_F(OEMCryptoSessionTestsDecryptEdgeCases, OddOffset) {
               subsample_size);
 }
 
+// This tests that the algorithm used to increment the counter for
+// AES-CTR mode is correct.  There are two possible implementations:
+// 1) increment the counter as if it were a 128 bit number,
+// 2) increment the low 64 bits as a 64 bit number and leave the high bits alone.
+// For CENC, the algorithm we should use is the second one.  OpenSSL defaults to
+// the first.  If this test is not passing, you should look at the way you
+// increment the counter.  Look at the example code in ctr128_inc64 above.
+// If you start with an IV of 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFE, after you
+// increment twice, you should get 0xFFFFFFFFFFFFFFFF0000000000000000.
 TEST_F(OEMCryptoSessionTestsDecryptEdgeCases, DecryptWithNearWrap) {
   vector<size_t> subsample_size;
   subsample_size.push_back(150);