Move Prerender* to platform/

BUG=297477

Review URL: https://codereview.chromium.org/48933002

git-svn-id: svn://svn.chromium.org/blink/trunk@160788 bbb929c8-8fbe-4397-9dbb-9b2b20218538
PseudoElement doesn't need to check confusingAndOftenMisusedAttached()

If a PseudoElement exists then it's attached. This code was confused.
Unfortunately there's nothing good to assert here except
confusingAndOftenMisusedAttached() which kind of defeats the purpose of
removing this caller.

Review URL: https://codereview.chromium.org/48993006

git-svn-id: svn://svn.chromium.org/blink/trunk@160789 bbb929c8-8fbe-4397-9dbb-9b2b20218538