Allow all app windows to minimize.

On CrOS, app windows can already be minimized by clicking their icon in the
taskbar. This just makes them show a minimize button as well.

On Windows, we prevent minimize if the window can't maximize, which
doesn't really make sense. They will now be able to minimize and show a
minimize button.

On Mac and Linux they could always minimize.

BUG=351179

Review URL: https://codereview.chromium.org/603523002

Cr-Commit-Position: refs/heads/master@{#296437}

components/native_app_window/native_app_window_views.cc

@@ -219,8 +219,7 @@ bool NativeAppWindowViews::CanMaximize() const {
 }
 
 bool NativeAppWindowViews::CanMinimize() const {
-  // TODO(jackhou): Change this to always be true.
-  return CanMaximize();
+  return true;
 }
 
 base::string16 NativeAppWindowViews::GetWindowTitle() const {