Replace third_party/gif with new DEPS entry.

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

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

  - rm src/codec/SkGifCodec.* third_party/gif/*

Original CL: https://review.skia.org/254582

Cq-Include-Trybots: luci.skia.skia.primary:Build-Debian9-Clang-x86-devrel-Android_SKQP
Bug: skia:9654
Change-Id: Ia0379af70a5ec5656a7d123b8b6816b9fa24eaee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255780
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 41587f8..0b4580a 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -27,7 +27,7 @@
 #ifdef SK_HAS_WUFFS_LIBRARY
 #include "src/codec/SkWuffsCodec.h"
 #elif defined(SK_USE_LIBGIFCODEC)
-#include "src/codec/SkGifCodec.h"
+#include "SkGifCodec.h"
 #endif
 
 struct DecoderProc {