Convert the re2 GYP targets to be generated by GN.

third_party/re2/BUILD.gn

Index: third_party/re2/BUILD.gn
diff --git a/tools/gn/secondary/third_party/re2/BUILD.gn b/third_party/re2/BUILD.gn
similarity index 94%
rename from tools/gn/secondary/third_party/re2/BUILD.gn
rename to third_party/re2/BUILD.gn
index e795db0eaa0f26b3c8dbfeda12df9eb775963845..8ae94a2ba67903b2622123f9be6cc3fbb104224a 100644
--- a/tools/gn/secondary/third_party/re2/BUILD.gn
+++ b/third_party/re2/BUILD.gn
@@ -2,13 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+gyp_file = "//out/gn_gyp/third_party/re2/re2.gyp"
+
 config("re2_config") {
   include_dirs = [ "." ]
 }
 
-gyp_file = "re2.gyp"
-external = true
-
 static_library("re2") {
   sources = [
     "mswin/stdint.h",
@@ -70,7 +69,6 @@ static_library("re2") {
 
   if (is_win) {
     include_dirs = [ "mswin" ]
-    # TODO(brettw) 'msvs_disabled_warnings': [ 4018, 4722, 4267 ],
   } else {
     sources -= "mswin/stdint.h"
   }