cppgc: Add public header-only target

The target can be used in Chromium to get basic cppgc types without
building the actual library.

Bug: chromium:1056170
Change-Id: Idaed19d265c63b2665e34d667903804708411c45
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2532308
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71136}

BUILD.gn

@@ -4386,6 +4386,23 @@ v8_source_set("v8_cppgc_shared") {
   public_deps = [ ":v8_libbase" ]
 }
 
+# This is split out to be a non-code containing target that the Chromium browser
+# can depend upon to get basic cppgc types.
+v8_header_set("cppgc_headers") {
+  configs = [ ":internal_config" ]
+  public_configs = [ ":v8_header_features" ]
+
+  sources = [
+    "include/cppgc/garbage-collected.h",
+    "include/cppgc/member.h",
+    "include/cppgc/persistent.h",
+    "include/cppgc/type-traits.h",
+    "include/cppgc/visitor.h",
+  ]
+
+  public_deps = [ ":v8_headers" ]
+}
+
 v8_source_set("cppgc_base") {
   visibility = [ ":*" ]