GN/ICU cleanup

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>
9 files changed