@Override
    public void stopNetLog() {
        synchronized (mLock) {
            if (!mIsLogging) {
                return;
            }
            checkHaveAdapter();
            mStopNetLogCompleted = new ConditionVariable();
            nativeStopNetLog(mUrlRequestContextAdapter);
            mIsLogging = false;
        }
        mStopNetLogCompleted.block();
    }

    @CalledByNative
    public void stopNetLogCompleted() {
        mStopNetLogCompleted.open();
    }

I imagine a hang looking like:
1. on thread #1: CronetEngine.startNetLog() called.
2. on thread #1: CronetEngine.stopNetLog() called, left waiting on mStopNetLogCompleted.block().
3. on thread #2: CronetEngine.startNetLog() called.
4. on thread #2: CronetEngine.stopNetLog() called, resets mStopNetLogCompleted to a new ConditionVariable.
Now thread #1 is left waiting forever.  stopNetLogCompleted() will open the second ConditionVariable created in step #4.