Don't call hb_buffer_set_unicode_funcs in HarfBuzzShaper

Harfbuzz-ng 0.9.27 introduced a HAVE_ICU_BUILTIN define which makes the
hb_buffer_set_unicode_funcs call redundant. Also call
hb_buffer_clear_content instead of hb_buffer_reset which is faster now
that we no longer explicitly set the unicode functions.

BUG=354117
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/197213032

git-svn-id: svn://svn.chromium.org/blink/trunk@169604 bbb929c8-8fbe-4397-9dbb-9b2b20218538