Remove toV8ForMainWorld and DOMDataStore::getWrapperForMainWorld.

This is a follow-up to https://codereview.chromium.org/19566002.

The perf-critical paths should already use setV8ReturnValueForMainWorld.

The only interesting places using DOMDataStore::getWrapperForMainWorld were in
V8Window; the rest of the usages had already been migrated.

BUG=236290

Review URL: https://chromiumcodereview.appspot.com/22894003

git-svn-id: svn://svn.chromium.org/blink/trunk@156306 bbb929c8-8fbe-4397-9dbb-9b2b20218538