If m_runs is empty we do not need to call reverseRuns. Additionally, verifies if we have any runs in BidiRunList before trying to reverse.

BUG=302572

Review URL: https://codereview.chromium.org/25373008

git-svn-id: svn://svn.chromium.org/blink/trunk@158752 bbb929c8-8fbe-4397-9dbb-9b2b20218538