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(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