Delete runs after use in RenderText::computePreferredLogicalWidths

Update RenderText::computePreferredLogicalWidths to delete runs once
they have been used for prefs width calculation thereby avoiding a
memory leak introduced by r164788.

R=leviw@chromium.org,dglazkov@chromium.org
BUG=333888

Review URL: https://codereview.chromium.org/136533004

git-svn-id: svn://svn.chromium.org/blink/trunk@164996 bbb929c8-8fbe-4397-9dbb-9b2b20218538