clang/win: Make everything work in a GN build.

tools/gn/docs/cookbook.md

 # GYP->GN Conversion Cookbook
 
 [TOC]
 
 ## Targets
 
 | *GYP*                                            | *GN*                                               |
 |:-------------------------------------------------|:---------------------------------------------------|
 | `'type': 'static_library', 'name': 'foo',`       | `static_library("foo") {` or `source_set("foo") {` |
 | `'type': 'shared_library', 'name': 'foo',`       | `shared_library("foo") {`                          |
@@ skipping 466 matching lines @@
 At least half of the warnings in these blocks are already disabled
 globally (we added more global ones later). From the command line, do:
 
 ```
 $ cd src/build/config
 $ git grep 4018
 compiler/BUILD.gn:      "/wd4018",  # Comparing signed and unsigned values.
 ```
 
 tells us that warning 4018 is already disabled globally from the
-`//build/config/compiler:default_warnings` config, and the same for
-4244. So ignore these.
+`default_warning_flags` variable in `//build/config/compiler`, and the same
+for 4244. So ignore these.
 
 Always comment what the warning is. Use your favorite search engine and
 type "vc warning 4267" to look it up. You'll end up with:
 
 ```
 if (is_win) {
   cflags += [
     "/wd4267",  # Conversion from size_t to 'type'.
   ]
 }
@@ skipping 144 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```