Fix loop condition in BidiRunList::replaceRunWithRuns

Fix the loop condition in BidiRunList::replaceRunWithRuns to check if a
next run exists before accessing it.

R=inferno@chromium.org
BUG=397479

Review URL: https://codereview.chromium.org/421383002

git-svn-id: svn://svn.chromium.org/blink/trunk@179068 bbb929c8-8fbe-4397-9dbb-9b2b20218538