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",
     ],