DevTools: Do not delete self in LifecycleObserver.contextDestroyed() callback in AsyncCallStackTracker.

We can not delete self in the contextDestroyed() callback due to design of the LifecycleObserver. Instead defer deleting destroyed context data.

Drive-by: Get rid of memcpy using HashMap.swap().

BUG=328366
R=yurys@chromium.org, pfeldman@chromium.org, yurys

Review URL: https://codereview.chromium.org/99053005

git-svn-id: svn://svn.chromium.org/blink/trunk@164012 bbb929c8-8fbe-4397-9dbb-9b2b20218538