Expand minimum size of the window to avoid cut off context menu

BUG=259656
TEST=manually

Review URL: https://codereview.chromium.org/510713002

Cr-Commit-Position: refs/heads/master@{#292348}

ui/file_manager/file_manager/background/js/background.js

@@ -614,7 +614,7 @@ var FILE_MANAGER_WINDOW_CREATE_OPTIONS = Object.freeze({
     height: Math.round(window.screen.availHeight * 0.8)
   }),
   minWidth: 480,
-  minHeight: 240,
+  minHeight: 300,
   hidden: true
 });