GN: use the correct fontconfig font mugger.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 8bf9d668cb664aef8f03b74701cabd92a6948cab..a310e33b8d45608ae0a3f3aaa0f1a3228c607231 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -257,12 +257,12 @@ component("skia") {
       "third_party:freetype2",
     ]
     sources += [
-      "src/fonts/SkFontMgr_fontconfig.cpp",

[bungeman-skia, 2016/08/01 20:03:17]

If this build file is designed for building and testing as many files as
possible, leaving this one in should be ok (otherwise we're only building it
when rolling into Chromium).

[mtklein, 2016/08/01 20:05:55]

Nah, quite the opposite.  For now I don't want anything that we don't need.

       "src/ports/SkFontConfigInterface_direct.cpp",
       "src/ports/SkFontConfigInterface_direct_factory.cpp",
       "src/ports/SkFontHost_FreeType.cpp",
       "src/ports/SkFontHost_FreeType_common.cpp",
-      "src/ports/SkFontHost_fontconfig.cpp",

[bungeman-skia, 2016/08/01 20:03:17]

This file should actually be named SkFontMgr_FCI_factory.cpp, and as such will
conflict with the other SkFontMgr_XXX_factory.cpp files.

+      "src/ports/SkFontMgr_fontconfig.cpp",
+      "src/ports/SkFontMgr_fontconfig_factory.cpp",
     ]
   }