Revert "Remove HarfBuzzRun cache"

Revert the change to remove the HarfBuzzRun cache as it had a larger
performance penalty than expected on Windows XP and Android on some
popular real world web sites. Looks like we'll have to implement a
proper word-cache before we can remove the existing cache.

Reverts the following changes:
  r180470 5b62ff8f7ce16acb5c3a0e156d7d7d78412a3d74 Shape across HarfBuzz runs
  r180408 8d067f53cc15011f649cb4c92ed2f549474a2faf Remove HarfBuzzRun cache

TBR=dglazkov@chromium.org
BUG=404717

Review URL: https://codereview.chromium.org/489993003

git-svn-id: svn://svn.chromium.org/blink/trunk@180695 bbb929c8-8fbe-4397-9dbb-9b2b20218538