For targets that depend on ICU, only define if `skia_use_icu` is set.
Move declare_args/skia_use_icu into skia.gni. Other variables that need
to work like that can move there later.
icu/BUILD.gn defines SK_USING_THIRD_PARTY_ICU if needed.
SkShaper_harfbuzz.cpp SkPDFSubsetFont.cpp respects
SK_USING_THIRD_PARTY_ICU when calling SkLoadICU().
sfntly/BUILD.gn, harfbuzz/BUILD.gn, icu/icu.gni uses $_src variable to
reduce verbosity. icu/icu.gni adds more headersto sources.
Change-Id: I9e000b9b19902d9f5c0c64e989bf42466aa8a299
Reviewed-on: https://skia-review.googlesource.com/c/189304
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
