Custom Element should have a wrapper while frame navigation

When frame navigation occurs, the script context containing
the custom element is destroyed. And, hence we were hitting
an early written.
This patch allocates the wrapper in whatever context the other
wrappers are being allocated in.

BUG=323430

Review URL: https://codereview.chromium.org/604773002

git-svn-id: svn://svn.chromium.org/blink/trunk@183064 bbb929c8-8fbe-4397-9dbb-9b2b20218538