[Paint Preview] Correct unparsable subframe field in proto

Protos that populate the subframe field aren't parsable once serialized
as the is_main_frame field isn't populated. This wasn't caught in any
tests and iframes aren't captured yet. I caught this when working on
some Cluster Telemetry metrics collection.

Bug: 1024640
Change-Id: I4632e07b165da3a47534081e6fa024573c5046ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1914512Reviewed-by: Mehran Mahmoudi <mahmoudi@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715278}

components/paint_preview/browser/paint_preview_client.cc

@@ -276,6 +276,7 @@ mojom::PaintPreviewStatus PaintPreviewClient::RecordFrame(
     frame_proto->set_is_main_frame(true);
   } else {
     frame_proto = proto_ptr->add_subframes();
+    frame_proto->set_is_main_frame(false);
   }
   // Safe since always <#>.skp.
   frame_proto->set_file_path(filename.AsUTF8Unsafe());