Avoid file IO on UI thread by moving more save-page-as stuff to the FILE thread.

base/nix/mime_util_xdg.h

Index: base/nix/mime_util_xdg.h
diff --git a/base/nix/mime_util_xdg.h b/base/nix/mime_util_xdg.h
index e40415e1c4800245cc6de8fa4ac41fdde476f987..41bbf6d28881d978c1c00fab03938a428d1cd782 100644
--- a/base/nix/mime_util_xdg.h
+++ b/base/nix/mime_util_xdg.h
@@ -16,9 +16,17 @@ class FilePath;
 
 namespace nix {
 
-// Gets the mime type for a file based on its filename. The file path does not
-// have to exist. Please note because it doesn't touch the disk, this does not
-// work for directories.
+// Gets the mime type for a file at |filepath|.
+//
+// The mime type is calculated based only on the file name of |filepath|.  In
+// particular |filepath| will not be touched on disk and |filepath| doesn't even
+// have to exist.  This means that the function does not work for directories
+// (i.e. |filepath| is assumed to be a path to a file).
+//
+// Note that this function might need to read from disk the mime-types data
+// provided by the OS.  Therefore this function should not be called from
+// threads that disallow IO via base::ThreadRestrictions::SetIOAllowed(false).
+//
 // If the mime type is unknown, this will return application/octet-stream.
 BASE_EXPORT std::string GetFileMimeType(const FilePath& filepath);