Add test for ChromeVox keyboard commands.
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}