Use ReadOnlyFile for memory usage statm and status files

These are opened FLAG_READ.

https://source.chromium.org/chromium/chromium/src/+/master:content/browser/renderer_host/render_process_host_impl.cc;drc=da8e399b1f509d29565224190204860b22c9062b;l=5202

Bug: 1130762
Change-Id: I5db1b78def792ff1ad6f801789e775101312ee51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2461051Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815855}

third_party/blink/public/mojom/memory_usage_monitor_linux.mojom

@@ -4,12 +4,13 @@
 
 module blink.mojom;
 
-import "mojo/public/mojom/base/file.mojom";
+import "mojo/public/mojom/base/read_only_file.mojom";
 
 // The interface to provide status and statm files to the renderer's memory
 // usage monitor. The interface is required only for Linux.
 interface MemoryUsageMonitorLinux {
   // The sandbox prevents the renderer from open()-ing any file, meaning
   // the file descriptors have to be provided by the browser process.
-  SetProcFiles(mojo_base.mojom.File statm_file, mojo_base.mojom.File status_file);
+  SetProcFiles(mojo_base.mojom.ReadOnlyFile statm_file,
+               mojo_base.mojom.ReadOnlyFile status_file);
 };