[GN]: Enable PCH for WebKit on Mac.

+ Add a new WebKit-level config "mac_precompiled_headers".
+ Add it as a public_config on core:prerequisites since the targets that need it depend on core:prerequisites.
+ Add it as a normal config on :platform since only the target itself needs it, not any deps.

BUG=none

Review URL: https://codereview.chromium.org/1397143003

Cr-Original-Commit-Position: refs/heads/master@{#353559}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 622249055279172f4b064ed2e0ace8aa0dadddc2

tools/gn/docs/cookbook.md

diff --git a/tools/gn/docs/cookbook.md b/tools/gn/docs/cookbook.md
index 3f2233c..7a69737 100644
--- a/tools/gn/docs/cookbook.md
+++ b/tools/gn/docs/cookbook.md
@@ -438,7 +438,7 @@
 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.