Revert "Replace third_party/gif with new DEPS entry."

This reverts commit 6d29bfbfcde968ec3ce9c9ddaf225336d5ab15e5.

Reason for revert: try to fix g3 roll.

Original change's description:
> 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>

TBR=djsollen@google.com,halcanary@google.com,rmistry@google.com,scroggo@google.com

Change-Id: I427e1a073c0574e1502b6196873ac3001a22250b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:9654
Cq-Include-Trybots: luci.skia.skia.primary:Build-Debian9-Clang-x86-devrel-Android_SKQP
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255534
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Hal Canary <halcanary@google.com>
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 0b4580a..cc5b9bf 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -26,8 +26,8 @@
 #include "src/codec/SkWebpCodec.h"
 #ifdef SK_HAS_WUFFS_LIBRARY
 #include "src/codec/SkWuffsCodec.h"
-#elif defined(SK_USE_LIBGIFCODEC)
-#include "SkGifCodec.h"
+#else
+#include "src/codec/SkGifCodec.h"
 #endif
 
 struct DecoderProc {
@@ -45,7 +45,7 @@
     #endif
     #ifdef SK_HAS_WUFFS_LIBRARY
         { SkWuffsCodec_IsFormat, SkWuffsCodec_MakeFromStream },
-    #elif defined(SK_USE_LIBGIFCODEC)
+    #else
         { SkGifCodec::IsGif, SkGifCodec::MakeFromStream },
     #endif
     #ifdef SK_HAS_PNG_LIBRARY