Pull up LayoutStateDisabler in updateFirstLetterStyle

Avoid improper repainting/asserting when falling into the
case in updateFirstLetterStyle where the style change doesn't
trigger a reattach. In that case, we'd still potentially
issue a repaint, but without the LayoutStateDisabler,
and with LayoutState set up improperly for the task.

BUG=379802

Review URL: https://codereview.chromium.org/311633002

git-svn-id: svn://svn.chromium.org/blink/trunk@175319 bbb929c8-8fbe-4397-9dbb-9b2b20218538