Removed parameter that is always true.

The dirty parameter to InlineBox::markDirty is now always true. Remove it.
Follow-up after landing https://codereview.chromium.org/254573006/

R=leviw@chromium.org

Review URL: https://codereview.chromium.org/257763004

git-svn-id: svn://svn.chromium.org/blink/trunk@172704 bbb929c8-8fbe-4397-9dbb-9b2b20218538