(This is a merge of http://go/wvgerrit/72764)
Netflix has identified a calling pattern that causes this mutex to be
taken recursively. This is not guaranteed to be safe for Widevine's
old custom Lock implementation nor std::mutex. However, it is guaranteed
to be safe for std::recursive_mutex. This patch updates the mutex in use
accordingly.
In the long-term, this lock needs to be reconsidered, as already noted
by comments in the code. It would be great if the reconsidered locking
did not require a recursive-safe lock. The TODO for this has been spun
off into its own bug and the comment has been updated to point to this.
Bug: 120471929
Test: CE CDM Unit Tests
Test: Android Unit Tests
Change-Id: I34df64456de4b469b75caf25a33f0bc53a5da330
85d8e961f8