Bring up the Fast User Switcher on Shift+Click on the new Avatar Button

BUG=403619

TEST
1. Enable the New Avatar Menu and Fast User Switcher flags in chrome://flags
2a. Left click on the new avatar button, the entire menu should appear (tutorials if applicable, current user avatar, list of profiles to switch to, options buttons)
2b. Right click on the new avatar button, only the list of available profiles should show up.

Review URL: https://codereview.chromium.org/869793004

Cr-Commit-Position: refs/heads/master@{#315643}
9 files changed