Share precompiled header setup for blink between Windows and Mac.

third_party/WebKit/Source/platform/BUILD.gn

Index: third_party/WebKit/Source/platform/BUILD.gn
diff --git a/third_party/WebKit/Source/platform/BUILD.gn b/third_party/WebKit/Source/platform/BUILD.gn
index a7bae8a220da1a0f47e31d384d1324fdcf61edfe..18a1dc2db7a7a74ed2f2919297522951e1885032 100644
--- a/third_party/WebKit/Source/platform/BUILD.gn
+++ b/third_party/WebKit/Source/platform/BUILD.gn
@@ -169,8 +169,7 @@ config("blink_platform_pch") {
       # This is a string rather than a file GN knows about. It has to match
       # exactly what's in the /FI flag below, and what might appear in the
       # source code in quotes for an #include directive.
-      precompiled_header =
-          rebase_path("win/Precompile-platform.h", root_build_dir)
+      precompiled_header = rebase_path("Precompile-platform.h", root_build_dir)
 
       # This is a file that GN will compile with the above header. It will be
       # implicitly added to the sources (potentially multiple times, with one
@@ -180,6 +179,9 @@ config("blink_platform_pch") {
 
       # Force include the header.
       cflags = [ "/FI$precompiled_header" ]
+    } else if (is_mac) {
+      precompiled_source =
+          "//third_party/WebKit/Source/platform/Precompile-platform.h"
     }
   }
 }
@@ -1460,7 +1462,6 @@ component("platform") {
       "text/LocaleICU.h",
     ]
 
-    configs += [ "//third_party/WebKit/Source:mac_precompiled_headers" ]
     libs = [
       "AppKit.framework",
       "Accelerate.framework",