Fix an outdated comment in external_filesystem_apitest.cc

TEST=n/a
BUG=none

Review URL: https://codereview.chromium.org/560753002

Cr-Commit-Position: refs/heads/master@{#294145}

chrome/browser/chromeos/file_manager/external_filesystem_apitest.cc

@@ -44,8 +44,7 @@
 //
 // The tests cover following scenarios:
 // - Performing file system operations on external file systems from an
-//   extension with fileManagerPrivate permission (i.e. a file browser
-//   extension).
+//   app with fileManagerPrivate permission (i.e. Files.app).
 // - Performing read/write operations from file handler extensions. These
 //   extensions need a file browser extension to give them permissions to access
 //   files. This also includes file handler extensions in filesystem API.