GitLab

Change the behavior of fast user switching on Mac from Command click to right
click to be consistent with Windows and Linux.

BUG=458755

TEST=
1. Disable #enable-fast-user-switcher and enable #new-avatar-menu flags in chrome://flags
2. Relaunch Chrome
3. Right click on the Avatar Button, the fast user switcher should be shown
4. Command+Click on the Avatar Button, nothing should happen

Review URL: https://codereview.chromium.org/916523003

Cr-Commit-Position: refs/heads/master@{#317631}