Remove an assert that triggers a bad cast in blink::LifecycleNotifier.

The bad cast happens in
blink::LifecycleNotifier<blink::ExecutionContext, blink::ContextLifecycleObserver>::context(),
when it's called from the destructor. See https://crbug.com/537398.

The primary issue is that this method makes a base-to-derived cast at the time,
when the derived instance is already destroyed.

This bug was found by Control Flow Integrity check:
https://sites.google.com/a/chromium.org/dev/developers/testing/control-flow-integrity

BUG=chromium:537398,chromium:464797

Review URL: https://codereview.chromium.org/1381823002

Cr-Commit-Position: refs/heads/master@{#351884}
1 file changed