Merge from widevine repo of http://go/wvgerrit/20660
For CTR encryption mode, when a subsample ends with partial block that
is continued on the next subsample, both blocks should have the same
IV. This allows an implementation of OEMCrypto to optimize their
decryption algorithm.
b/31114392
Change-Id: I29a998b00f3bfb12c4bbbcb1fa1ebc371473fefd