DevTools: Do not invoke HashMap.get(NULL) in AsyncCallStackTracker.

The contextDestroyed method of the LifecycleObserver can pass
a (valid) ExecutionContext* NULL pointer.
Do not do a lookup into the HashMap in this case.

R=yurys@chromium.org, yurys

Review URL: https://codereview.chromium.org/109613008

git-svn-id: svn://svn.chromium.org/blink/trunk@163790 bbb929c8-8fbe-4397-9dbb-9b2b20218538