Implement 'action' in gn_to_cmake.py.

With this change it is possible to build all Skia targets.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2784

Change-Id: I5c53a6ba5746d210e0ae8f7a29b409d40c7b9bff
Reviewed-on: https://skia-review.googlesource.com/2784
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index d7cdec6..53f49ec 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -576,6 +576,7 @@
   script = "gn/echo_headers.py"
   args = [ rebase_path("$target_gen_dir/skia.h", root_build_dir) ] +
          rebase_path(skia_h_headers, target_gen_dir)
+  inputs = skia_h_headers
   outputs = [
     "$target_gen_dir/skia.h",
   ]