GN: fontmgr_custom

This makes skia_use_fontconfig=false build on Linux.  Instead of using
fontconfig it looks in /usr/share/fonts.

That alone is a nice feature to have, but it's mostly relevant for MSAN,
where we can't link against any uninstrumented system libraries.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2318133003

Review-Url: https://codereview.chromium.org/2318133003
diff --git a/BUILD.gn b/BUILD.gn
index 1b90048..5ea18dd 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -224,6 +224,18 @@
   ]
 }
 
+optional("fontmgr_custom") {
+  enabled = is_linux && skia_use_freetype && !skia_use_fontconfig
+
+  deps = [
+    "//third_party/freetype2",
+  ]
+  sources = [
+    "src/ports/SkFontMgr_custom.cpp",
+    "src/ports/SkFontMgr_custom_directory_factory.cpp",
+  ]
+}
+
 optional("fontmgr_fontconfig") {
   enabled = skia_use_freetype && skia_use_fontconfig
 
@@ -347,6 +359,7 @@
     ":crc32",
     ":dsp",
     ":fontmgr_android",
+    ":fontmgr_custom",
     ":fontmgr_fontconfig",
     ":gif",
     ":jpeg",