[Views] Don't try and show a context menu for the root menu item view
[Views] Don't try and show a context menu for the root menu item view

Don't show a context menu for the root menu item view.  The root view is
boundless and invisible to the user, but is implicitly selected when a menu
is first opened. If the user then clicks on the menu/"apps" key, the menu
controller would try and show a menu - which shows up at screen location 0,0 and
is typically unexpected behavior. Since there's not really a use case for
showing a context menu on the root menu, don't try to.

BUG=510343

Review URL: https://codereview.chromium.org/1241963002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339536}

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