(This is a merge of http://go/wvgerrit/14242)
To try to fix b/17761616, I added a lock to the DRM Plugin. However,
the API guarantees that any single DrmPlugin will never be called into
from multiple threads at the same time, rendering this unnecessary.
Previously, b/18194977 meant that this was not always true, but now
that that bug is fixed, this lock is not needed and can be removed.
Bug: 18194977
Change-Id: I25ee5fa44f35706f863b036997453441eac29c95