Fix creating InjectedScript too early causing extension bindings not to work.

BUG=263162
TEST=manual

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=155947

Review URL: https://chromiumcodereview.appspot.com/22884002

git-svn-id: svn://svn.chromium.org/blink/trunk@156062 bbb929c8-8fbe-4397-9dbb-9b2b20218538