protected boolean cleanUpSession(CustomTabsSessionToken sessionToken) {
        try {
            synchronized (mDeathRecipientMap) {
                IBinder binder = sessionToken.getCallbackBinder();
                DeathRecipient deathRecipient =
                        mDeathRecipientMap.get(binder);
                binder.unlinkToDeath(deathRecipient, 0);
                mDeathRecipientMap.remove(binder);
            }
        } catch (NoSuchElementException e) {
            return false;
        }
        return true;
    }
Called when the client side {@link IBinder} for this {@link CustomTabsSessionToken} is dead.
Can also be used to clean up {@link DeathRecipient} instances allocated for the given token.

@param sessionToken The session token for which the {@link DeathRecipient} call has been
                    received.
@return Whether the clean up was successful. Multiple calls with two tokens holdings the
same binder will return false.
customtabs/src/android/support/customtabs/CustomTabsService.java