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This is just a couple of quick sanity checks to prevent future regressions
like the one in the linked bug. It tests both a Search+Shift shortcut and
the corresponding Prefix Key shortcut.

Adds support for sending the meta modifier key in
aura/x11 tests.

BUG=404470

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=291547

Committed: https://chromium.googlesource.com/chromium/src/+/8cab2737f1cb6e6011b0946f6bef5027868602c0

Review URL: https://codereview.chromium.org/490443002

Cr-Commit-Position: refs/heads/master@{#293687}