Adds GN build files for building in GN based projects.

runtime/third_party/double-conversion/src/BUILD.gn

Index: runtime/third_party/double-conversion/src/BUILD.gn
===================================================================
--- runtime/third_party/double-conversion/src/BUILD.gn	(revision 0)
+++ runtime/third_party/double-conversion/src/BUILD.gn	(revision 0)
@@ -0,0 +1,32 @@
+# Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+
+static_library("libdouble_conversion") {
+  configs += [ "../../../..:dart_config", ]
+  sources = [
+    "bignum.cc",
+    "bignum.h",
+    "bignum-dtoa.cc",
+    "bignum-dtoa.h",
+    "cached-powers.cc",
+    "cached-powers.h",
+    "diy-fp.cc",
+    "diy-fp.h",
+    "double-conversion.cc",
+    "double-conversion.h",
+    "fast-dtoa.cc",
+    "fast-dtoa.h",
+    "fixed-dtoa.cc",
+    "fixed-dtoa.h",
+    "ieee.h",
+    "strtod.cc",
+    "strtod.h",
+    "utils.h",
+  ]
+  include_dirs = [
+    ".",
+  ]
+}
+