Moves cpp_headers into public_deps when depending on the dawn:cpp.

Bug: dawn:1822
Change-Id: Iaec2efe2349c48a0dbbd2bb4e043e0ff07795a60
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/133482
Reviewed-by: Austin Eng <enga@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Loko Kung <lokokung@google.com>

src/dawn/BUILD.gn

@@ -27,10 +27,8 @@ dawn_json_generator("cpp_gen") {
 }
 
 source_set("cpp") {
-  deps = [
-    ":cpp_gen",
-    "${dawn_root}/include/dawn:cpp_headers",
-  ]
+  public_deps = [ "${dawn_root}/include/dawn:cpp_headers" ]
+  deps = [ ":cpp_gen" ]
   sources = get_target_outputs(":cpp_gen")
 }