linux: Add tests for FontRenderParams on Linux.

Add some tests for FontRenderParams's interactions with
Fontconfig, which is used on desktop Linux and Chrome OS.

Also re-add platform_font_pango_unittest.cc for gn.

BUG=125235

Review URL: https://codereview.chromium.org/394963002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@283969 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

config/linux/BUILD.gn

@@ -47,6 +47,10 @@ pkg_config("pangocairo") {
   packages = [ "pangocairo" ]
 }
 
+pkg_config("pangoft2") {
+  packages = [ "pangoft2" ]
+}
+
 pkg_config("udev") {
   packages = [ "libudev" ]
 }