gn_utils: Export proto import_dirs for all targets

Right now proto import_dirs are only exported for source_set targets.

In a future commit, import_dirs will be used by zero targets as well.

This cl renames `metadata.import_dirs` to `metadata.proto_import_dirs`.
I haven't found a way in GN to set `metadata.import_dirs` on a target
that already has `import_dirs`.

Change-Id: Ifbfe087cb16956d625c72a0f81c585c25768b56d
diff --git a/Android.bp b/Android.bp
index e435f99..04d53c2 100644
--- a/Android.bp
+++ b/Android.bp
@@ -5323,6 +5323,7 @@
 genrule {
     name: "perfetto_protos_perfetto_metrics_webview_descriptor",
     srcs: [
+        ":libprotobuf-internal-descriptor-proto",
         "protos/perfetto/metrics/android/ad_services_metric.proto",
         "protos/perfetto/metrics/android/android_blocking_call.proto",
         "protos/perfetto/metrics/android/android_blocking_calls_cuj_metric.proto",
@@ -5388,7 +5389,7 @@
     tools: [
         "aprotoc",
     ],
-    cmd: "mkdir -p $(genDir)/external/perfetto/ && $(location aprotoc) --proto_path=external/perfetto --descriptor_set_out=$(out) $(in)",
+    cmd: "mkdir -p $(genDir)/external/perfetto/ && $(location aprotoc) --proto_path=external/perfetto --proto_path=external/protobuf/src --descriptor_set_out=$(out) $(in)",
     out: [
         "perfetto_protos_perfetto_metrics_webview_descriptor.bin",
     ],
diff --git a/gn/proto_library.gni b/gn/proto_library.gni
index f69d3ce..a7a77c6 100644
--- a/gn/proto_library.gni
+++ b/gn/proto_library.gni
@@ -371,7 +371,7 @@
 
         metadata = {
           proto_library_sources = invoker.sources
-          import_dirs = import_dirs_
+          proto_import_dirs = import_dirs_
           exports = get_path_info(public_deps_, "abspath")
         }
         forward_variables_from(invoker, vars_to_forward)
diff --git a/gn/standalone/proto_library.gni b/gn/standalone/proto_library.gni
index 07b8140..1a23a97 100644
--- a/gn/standalone/proto_library.gni
+++ b/gn/standalone/proto_library.gni
@@ -170,6 +170,10 @@
       ]
     }
 
+    metadata = {
+      proto_import_dirs = import_dirs
+    }
+
     if (generate_cc) {
       cc_generator_options_ = ""
       if (defined(invoker.cc_generator_options)) {
diff --git a/tools/gn_utils.py b/tools/gn_utils.py
index d0417d7..904760e 100644
--- a/tools/gn_utils.py
+++ b/tools/gn_utils.py
@@ -529,9 +529,8 @@
     return metadata.get('exports', [])
 
   def get_proto_paths(self, proto_desc):
-    # import_dirs in metadata will be available for source_set targets.
     metadata = proto_desc.get('metadata', {})
-    return metadata.get('import_dirs', [])
+    return metadata.get('proto_import_dirs', [])
 
   def get_proto_target_type(self, target: Target
                            ) -> Tuple[Optional[str], Optional[Dict]]: