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Most noticeably, always use the GTK-style menu layout on Desktop Linux.

Previously, the MenuConfig used by a particular MenuItemView could be
affected by that View's Widget's GetNativeTheme().

This once allowed a menu to decide whether to use a ChromeOS-style menu
or a native Desktop-style menu config on the same platform. But that's
no longer needed, and makes trouble with the "Classic Theme" in Desktop
Linux. (e.g. because menus are not always in a Widget when they are
configured, so the native theme is hard to determine.)

Using a single instance simplifies things and fixes a regression where
the Ash-style was being used on Desktop Linux, resulting in menus
without a border and unexpectedly large item padding. Regression due to
the combination of r344651, r345479, and r360168.

Note this means --open-ash on Desktop Linux, and Metro mode on Windows,
will use non-Ash menus. See screenshots at http://crbug.com/544550#c13

BUG=415953, 544550

Committed: https://crrev.com/5d23f8c18f1d9fbf1655ece40b31d59dea443c60
Cr-Commit-Position: refs/heads/master@{#363924}

Review URL: https://codereview.chromium.org/1477253002

Cr-Commit-Position: refs/heads/master@{#364577}