Add SK_ADDING_src_ports_SkFontConfigInterface_cpp.

A few method definitions in Skia need to be moved into a new cpp file,
while this define is active the old code will continue to compile until
the Chromium build of Skia can be updated.

NOTRY=true
Adds a define. Built locally, but analyze says "No compile necessary".

Review-Url: https://codereview.chromium.org/2355563002
Cr-Commit-Position: refs/heads/master@{#419506}

skia/BUILD.gn

@@ -48,7 +48,10 @@ config("skia_config") {
     "//third_party/skia/include/utils",
   ]
 
-  defines = [ "SK_IGNORE_DW_GRAY_FIX" ]
+  defines = [
+    "SK_IGNORE_DW_GRAY_FIX",
+    "SK_ADDING_src_ports_SkFontConfigInterface_cpp",
+  ]
   defines += skia_for_chromium_defines
 
   defines += []