Use ReadOnlyFile for network context file upload

These are opened readonly and there is no reason for the network process
to be able to write these files.

e.g.: https://source.chromium.org/chromium/chromium/src/+/master:content/browser/network_context_client_base_impl.cc;drc=0e22286ee95d2bd711f81d73f1178343fbacc890;l=33

Bug: 1130762
Change-Id: Ife99f494308f2a21286012cdbdfdd45e2b9de5ad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2464071
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817788}

services/network/public/mojom/network_context.mojom

@@ -5,8 +5,8 @@
 module network.mojom;
 
 import "mojo/public/mojom/base/big_buffer.mojom";
-import "mojo/public/mojom/base/file.mojom";
 import "mojo/public/mojom/base/file_path.mojom";
+import "mojo/public/mojom/base/read_only_file.mojom";
 import "mojo/public/mojom/base/time.mojom";
 import "mojo/public/mojom/base/unguessable_token.mojom";
 import "mojo/public/mojom/base/values.mojom";
@@ -814,7 +814,7 @@ interface NetworkContextClient {
   OnFileUploadRequested(int32 process_id,
                         bool async,
                         array<mojo_base.mojom.FilePath> file_paths) =>
-                        (int32 net_error, array<mojo_base.mojom.File> files);
+      (int32 net_error, array<mojo_base.mojom.ReadOnlyFile> files);
 
   // Checks if network error reports could be sent for the given origins.
   // Replies with the origins that are allowed.