Convert miscellaneous RenderThreadImpl messages to mojom

content/common/native_types.mojom

Index: content/common/native_types.mojom
diff --git a/content/common/native_types.mojom b/content/common/native_types.mojom
index 222ee34803ab995e8d3b7f84ac41373996814daf..9a8d9cd9aab259c1701f927e0645190cb072504b 100644
--- a/content/common/native_types.mojom
+++ b/content/common/native_types.mojom
@@ -20,5 +20,18 @@ struct RendererPreferences;
 [Native]
 struct ResizeParams;
 
+// NOTE: This type is only mapped and usable on Mac.
+[Native]
+enum ScrollbarButtonsPlacement;
+
+// NOTE: This type is only mapped and usable on Mac.
+[Native]
+enum ScrollerStyle;
+
 [Native]
 struct WebPreferences;
+
+// TODO(rockot): This should most likely be defined by network service mojom,
+// but the network service doesn't exist yet.
+[Native]
+enum NetworkConnectionType;