Fix Windows build warning LNK4217 for blink_platform.dll.

third_party/WebKit/Source/platform/mojo/BUILD.gn

Index: third_party/WebKit/Source/platform/mojo/BUILD.gn
diff --git a/third_party/WebKit/Source/platform/mojo/BUILD.gn b/third_party/WebKit/Source/platform/mojo/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..b47dfa9c194ff7644cb99b2c64ae8aed6c70dafb
--- /dev/null
+++ b/third_party/WebKit/Source/platform/mojo/BUILD.gn
@@ -0,0 +1,15 @@
+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("geometry_struct_traits") {
+  sources = [
+    "GeometryStructTraits.cpp",
+    "GeometryStructTraits.h",
+  ]
+  public_deps = [
+    "//third_party/WebKit/public:blink_headers",
+    "//ui/gfx/geometry",
+    "//ui/gfx/geometry/mojo:mojo_shared_cpp_sources",
+  ]
+}