Make a microtask of distribution recalc happen only when ShadowDOMV1 flag is enabled.

This is a follow-up patch of r376090. We do not need to enqueue a microtask if v1 is disabled.

Thus, having POC of 'slotchange' event for v1 will be zero performance impact for v0 world.

BUG=531990

Review URL: https://codereview.chromium.org/1715623002

Cr-Commit-Position: refs/heads/master@{#376480}
1 file changed