The existing deadlock detector code uses the trace-malloc stack walking
facilities, which is problematic for a few reasons.
1. It is only available in builds with --enable-trace-malloc, which is
not in the default build configuration.
2. It tries to capture a symbolicated stack trace every time that a lock
is acquired or released, which is really slow.
This patch changes the deadlock detector to use the XPCOM stack walking
and symbolification facilities, and avoids the symbolification until the
point where we need to print out the call stack, which makes the
deadlock detector a lot faster than it currently is in trace-malloc
builds.
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