Break HarfBuzz run creation into separate function

Break out creation of HarfBuzzRuns out of the collectHarfBuzzRuns
method as a part of the effort to clean up the logic to allow proper
handling of script_common.

BUG=311372
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/140933004

git-svn-id: svn://svn.chromium.org/blink/trunk@165253 bbb929c8-8fbe-4397-9dbb-9b2b20218538