[GN]: Enable PCH for WebKit on Mac.

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 420 matching lines @@
   'defines': [
     'FOO',
   ],
 },
 ```
 
 Note that many of the includes are trying to add the root "src"
 directory to the include path. This is always present in GN so you can
 remove these.
 
-GYP also requires you do duplicate these settings, once for the target
+GYP also requires you to duplicate these settings, once for the target
 itself, and once for the direct/all dependent settings. In GN,
 public/all dependent configs also apply to the current target so you
 only need to specify it once.
 
 In GN, put the settings in a config (declared above your target), and
 then reference that as a public config in your target:
 
 ```
 config("foo_config") {
   include_dirs = [ "." ]
@@ skipping 190 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```