Remove an obsolete comment in FrameView::performPostLayoutTasks.

FontFaceSet::didLayout doesn't execute JavaScript. It just kicks
AsyncMethodRunner.

BUG=315979
NOTRY=true

Review URL: https://codereview.chromium.org/93833002

git-svn-id: svn://svn.chromium.org/blink/trunk@162820 bbb929c8-8fbe-4397-9dbb-9b2b20218538