Defer setting "internalinstanceid" so that pending (and destroyed)
browser plugins can detach before we attach again.

BUG=467614
Test=Load a <webview>
Set its style.display = 'block'.
Observer that it succeeds without <webview> going stale or
throwing any error about "attach" failing.

Review URL: https://codereview.chromium.org/1015443002

Cr-Commit-Position: refs/heads/master@{#320803}
4 files changed