Revert "Revert "Experimental SkVideoDecoder, to wrap calls to ffmpeg""

fix google3 build (don't build the new gm)

This reverts commit 9cc53fa4bdf2c5c52463b8bf932a01fe04ff6d82.

Change-Id: I3de8aa4959f62a380f1d4aaf2b788d250775e194
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214442
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 87075af..42388de 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -21,6 +21,7 @@
   skia_use_angle = false
   skia_use_egl = false
   skia_use_expat = true
+  skia_use_ffmpeg = false
   skia_use_fontconfig = is_linux
   skia_use_fonthost_mac = is_mac
   skia_use_freetype = is_android || is_fuchsia || is_linux
@@ -1622,6 +1623,20 @@
     ]
   }
 
+  if (skia_use_ffmpeg) {
+    test_lib("video_decoder") {
+      sources = [
+        "experimental/ffmpeg/SkVideoDecoder.cpp",
+        "experimental/ffmpeg/SkVideoDecoder.h",
+      ]
+      libs = [
+        "avcodec",
+        "avformat",
+        "avutil",
+      ]
+    }
+  }
+
   import("gn/gm.gni")
   test_lib("gm") {
     sources = gm_sources
@@ -1638,6 +1653,11 @@
     public_deps = [
       ":gpu_tool_utils",
     ]
+
+    if (skia_use_ffmpeg) {
+      deps += [ ":video_decoder" ]
+      sources += [ "gm/video_decoder.cpp" ]
+    }
   }
 
   import("gn/tests.gni")