Add a port of hb_icu_script_to_script to HarfbuzzShaper
Add a port of hb_icu_script_to_script to HarfbuzzShaper

Harfbuzz on CrOS does not have hb-icu leading to a compile failure
when system_harfbuzz is turned on to avoid a linker warning.
(see http://crbug.com/353127).

HarfbuzzShaper uses just one function (hb_icu_script_to_script) from
hb-icu and it's a simple wrapper. It's ported to HarfbuzzShaper to avoid
the dependency on hb-icu.

BUG=356929
TEST=content_shell gets built on Linux and no change in layout tests.

Review URL: https://codereview.chromium.org/214413002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@170276 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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