Remove HarfBuzzRun cache

Remove the HarfBuzzRun cache as using the run text value as the cache
isn't good enough when shaping across runs (support for which is coming
in a separate change) or when text rendering settings have changed (CSS
font-feature-settings).
Early testing indicates that the performance implications of removing
the cache are negligible as the overhead of maintaining the cache
negates most of the savings.

R=behdad@chromium.org,leviw@chromium.org
BUG=345401
TEST=PerformanceTests/Layout/hindi-line-layout.html

Review URL: https://codereview.chromium.org/167703008

git-svn-id: svn://svn.chromium.org/blink/trunk@180408 bbb929c8-8fbe-4397-9dbb-9b2b20218538