Separate GifModule from ModuleMgr.

Then make GifModule methods static, and separate out the non-static part
into GifProgressiveDecoder.

Change-Id: Ifac98c3b72fed01e407466ca023e09aaa4c16c95
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/69934
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/core/fxcodec/BUILD.gn b/core/fxcodec/BUILD.gn
index b168112..bc478ae 100644
--- a/core/fxcodec/BUILD.gn
+++ b/core/fxcodec/BUILD.gn
@@ -107,6 +107,8 @@
         "gif/cfx_gifcontext.h",
         "gif/cfx_lzwdecompressor.cpp",
         "gif/cfx_lzwdecompressor.h",
+        "gif/gif_progressive_decoder.cpp",
+        "gif/gif_progressive_decoder.h",
         "gif/gifmodule.cpp",
         "gif/gifmodule.h",
       ]