Allow text autosizer to repaint during performLayout.

The performLayout method has two sections where the actual layout() is done.
Between those two sections it's possible for the text autosizer to set style
on a render object which will trigger a repaint(). This CL explicitly enables
this behaviour by wrapping the autosizer call in an AllowRepaintScope.

BUG=320139

Review URL: https://codereview.chromium.org/246403011

git-svn-id: svn://svn.chromium.org/blink/trunk@172413 bbb929c8-8fbe-4397-9dbb-9b2b20218538