Make sure a pure list of headers is never turned into a static library

Windows won't create empty .lib files, and will subsequently fail to
find the library other rules expect

R=machenbach@chromium.org
BUG=

Change-Id: I8b848834a6957c2164f0eafc853587d39ccb6ca9
Reviewed-on: https://chromium-review.googlesource.com/440244Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43141}

BUILD.gn

@@ -879,7 +879,7 @@ if (v8_use_external_startup_data) {
 
 # This is split out to be a non-code containing target that the Chromium browser
 # DLL can depend upon to get only a version string.
-v8_source_set("v8_version") {
+v8_header_set("v8_version") {
   configs = [ ":internal_config" ]
 
   sources = [

gni/v8.gni

@@ -112,6 +112,15 @@ template("v8_source_set") {
   }
 }
 
+template("v8_header_set") {
+  source_set(target_name) {
+    forward_variables_from(invoker, "*", [ "configs" ])
+    configs += invoker.configs
+    configs -= v8_remove_configs
+    configs += v8_add_configs
+  }
+}
+
 template("v8_executable") {
   executable(target_name) {
     forward_variables_from(invoker,