Make <dialog> EnabledAtRuntime rather than EnabledPerContext

This fixes the perf regression described in the bug.

I think it was only EnabledPerContext to support enabling dialog via
internals.settings, but that is no longer done (r154817).

BUG=272222

Review URL: https://chromiumcodereview.appspot.com/22815026

git-svn-id: svn://svn.chromium.org/blink/trunk@156558 bbb929c8-8fbe-4397-9dbb-9b2b20218538