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This matches other macOS application like Safari, Terminal, etc. All existing
hotkeys will still work.

This CL uses the same hotkey for "previous tab" that Safari and Terminal use,
which is [ctr + shift + "Horizontal Tab"]. This also causes tests to pass.
However, pressing that key combination actually generates [ctr + shift + "End of
Medium"], which renderers in the Main Menu as a backwards tab. This CL updates
NSMenuItem(ChromeAdditions) to check for this special case.

This CL removes two incorrect DCHECKs from render_widget_host_view_cocoa.mm. I
regularly hit both of them when running Chromium on a local build. They both
rely on the false assumption that keyEquivalents in the main menu must have the
command modifier.

Bug: 851714
Change-Id: I90c643870f9bc7dd87369496eec9ab62a1685e07
Reviewed-on: https://chromium-review.googlesource.com/1106659
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574211}