Implement main blink target in GN.

Source/BUILD.gn

Index: Source/BUILD.gn
diff --git a/Source/BUILD.gn b/Source/BUILD.gn
index 002dabbcd93639a0d150dac6265327751db860ee..f79fb727e12a6b7416d3cb598ccb569adeaffe3c 100644
--- a/Source/BUILD.gn
+++ b/Source/BUILD.gn
@@ -7,6 +7,8 @@ if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
+visibility = "//third_party/WebKit/*"
+
 # features ---------------------------------------------------------------------
 
 config("features") {
@@ -78,11 +80,3 @@ config("non_test_config") {
     cflags += [ "-Wglobal-constructors" ]
   }
 }
-
-# stubs ------------------------------------------------------------------------
-
-# These set up include paths for targets that haven't been converted yet. They
-# can be removed when the corresponding targets have been converted to GN.
-config("npapi_stub") {
-  include_dirs = [ "//third_party/npapi", "//third_party/npapi/bindings" ]
-}