Replace third_party/gif with new DEPS entry.

  - skia_libgifcodec_path gn var. Default is
    third_party/externals/libgifcodec.  Anfroid framework uses
    third_party/libgifcodec.

  - skia_use_libgifcodec gn var; defaults to !skia_use_wuffs.  Allow
    disabling both codecs.

  - BUILD.gn `gif` optional source_set: use new skia_libgifcodec_path
    code, not third_party/gif code.

  - GN template("optional"): add public_include_dirs, similar to
    public_defines.

  - SK_USE_LIBGIFCODEC, ifdef for skia_use_libgifcodec.

  - SkCodec.cpp: use third_party version of SkGifCodec.h.

  - src/codec/SkGifCodec.*, third_party/gif/*: remove files.

Cq-Include-Trybots: luci.skia.skia.primary:Build-Debian9-Clang-x86-devrel-Android_SKQP
Bug: skia:9654
Change-Id: Ie22d2bb7e52322fd3eedb97015f3aec2ce5e3402
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254582
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
10 files changed