Add a flag for selecting the ICU data source on Fuchsia

Introduces a `declare_args()` of `icu_fuchsia_override_data_dir` which
allows the builds of Fuchsia to select an alternate `data_dir` for CLDR
data source.

Most of the time, this flag value remains at its default setting, except
for some special Fuchsia builds which need to use their own.

Change-Id: Ifc5c34831454b164e77652d1d7239f906b3255c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/3566498
Reviewed-by: Frank Tang <ftang@chromium.org>
2 files changed