[Mac] Always use the small, non-high res default avatars in the People menu bar

BUG=447311
TEST=Start Chrome. Click on the "People" menu in the OS menu bar. Any profiles
with default avatar should display the low res, transparent background images
(i.e. not the high res images with the grey background)

Review URL: https://codereview.chromium.org/882113003

Cr-Commit-Position: refs/heads/master@{#313544}

chrome/browser/ui/cocoa/profiles/profile_menu_controller.mm

@@ -133,7 +133,13 @@ class Observer : public chrome::BrowserListObserver,
     if (dock) {
       [item setIndentationLevel:1];
     } else {
-      gfx::Image itemIcon = itemData.icon;
+      gfx::Image itemIcon;
+      bool isRectangle;
+      // Always use the low-res, small default avatars in the menu.
+      AvatarMenu::GetImageForMenuButton(itemData.profile_path,
+                                        &itemIcon,
+                                        &isRectangle);
+
       // The image might be too large and need to be resized (i.e. if this is
       // a signed-in user using the GAIA profile photo).
       if (itemIcon.Width() > profiles::kAvatarIconWidth ||