Windows precompiled header support in GN

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 9fc86fd2703a36aa877d146a9b158b900e94a577..acd02d7e0799b1ab7cd5532503cbf91635bb8f91 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -396,3 +396,21 @@ config("default_libs") {
     libs = [ "dl" ]
   }
 }
+
+config("precompiled_headers") {
+  if (is_win) {
+  #if (false) {
+    # 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 = "build/precompile.h"
+
+    # 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
+    # variant for each language used in the target).
+    precompiled_source = "//build/precompile.cc"
+
+    # Force include the header.
+    cflags = [ "/FI$precompiled_header" ]
+  }
+}