When updating a font family setting, only mark style and
layout as dirty if the font family was actually changed.

BUG=390799

Review URL: https://codereview.chromium.org/371853006

git-svn-id: svn://svn.chromium.org/blink/trunk@177607 bbb929c8-8fbe-4397-9dbb-9b2b20218538