Add initial BUILD.gn in platform/wtf/, and move first a few files there.

third_party/WebKit/Source/BUILD.gn

 # Copyright 2014 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.
 
 import("config.gni")
 if (is_clang) {
   import("//build/config/clang/clang.gni")
 }
 
 visibility = [ "//third_party/WebKit/*" ]
@@ skipping 59 matching lines @@
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
     }
   }
 }
 
 # config -----------------------------------------------------------------------
 
 config("config") {
   include_dirs = [
+    # crbug.com/691465: Temporary solution to fix an issue related to MSVC-
+    # specific #include search order. This essentially lets
+    # #include "wtf/Foo.h" actually include "platform/wtf/Foo.h". This is okay
+    # because these files are equivalent if both exist.

[Nico, 2017/02/24 15:17:51]

i kind of liked my comment better, but oh well :-)

+    "platform",
     ".",
     "..",
     "$root_gen_dir/blink",
     "$root_gen_dir/third_party/WebKit",
   ]
 
   cflags = []
   defines = []
 
   if (is_win) {
@@ skipping 74 matching lines @@
   }
 }
 
 if (is_mac) {
   # This sets up precompiled headers for Mac.
   config("mac_precompiled_headers") {
     precompiled_header = rebase_path("build/mac/Prefix.h", root_build_dir)
     precompiled_source = "//third_party/WebKit/Source/build/mac/Prefix.h"
   }
 }