Implement main blink target in GN.

public/blink.gyp

Index: public/blink.gyp
diff --git a/public/blink.gyp b/public/blink.gyp
index 8536c579ae5ea0ceb4be8a97a995b80329c74d4e..a389b2185cbf698dbfe9db84ba529514a3bc0a1f 100644
--- a/public/blink.gyp
+++ b/public/blink.gyp
@@ -33,6 +33,7 @@
     ],
     'targets': [
         {
+            # GN version: //third_party/WebKit/public:blink
             'target_name': 'blink',
             'type': 'none',
             'dependencies': [
@@ -52,6 +53,8 @@
             # places that cannot link against the full Blink library.
             # FIXME: We really shouldn't have this at all and should instead remove all uses
             # of Blink's API types from places that can't link against Blink. crbug.com/248653
+            #
+            # GN version: //third_party/WebKit/public:blink_minimal
             'target_name': 'blink_minimal',
             'type': 'none',
             'dependencies': [
@@ -62,6 +65,7 @@
             ],
         },
         {
+            # GN version: //third_party/WebKit/public:test_support
             'target_name': 'blink_test_support',
             'type': 'none',
             'dependencies': [