Defer style recalc when inserting extension stylesheets.

StyleEngine::addAuthorSheet, used when injecting stylesheets from extensions,
was doing a synchronous style recalc. Adding two stylesheets in a row would
cause two style recalcs. Do a deferred style recalc instead.

BUG=374901,374162

Review URL: https://codereview.chromium.org/290563007

git-svn-id: svn://svn.chromium.org/blink/trunk@174424 bbb929c8-8fbe-4397-9dbb-9b2b20218538