Use ReadOnlyFile for spellcheck dictionaries

These are opened read-only, this should ensure they stay that way.

https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/spellchecker/spellcheck_hunspell_dictionary.cc;drc=da8e399b1f509d29565224190204860b22c9062b;l=381

Bug: 1130762
Change-Id: I8086eeb824c895416074870196df0a01ff1c89ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462631
Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817798}

components/spellcheck/common/spellcheck.mojom

@@ -4,7 +4,7 @@
 
 module spellcheck.mojom;
 
-import "mojo/public/mojom/base/file.mojom";
+import "mojo/public/mojom/base/read_only_file.mojom";
 import "mojo/public/mojom/base/string16.mojom";
 
 // Render process interface exposed to the browser for receiving process-
@@ -25,7 +25,7 @@ interface SpellChecker {
 };
 
 struct SpellCheckBDictLanguage {
-  mojo_base.mojom.File? file;
+  mojo_base.mojom.ReadOnlyFile? file;
   string language;
 };