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On Mac, interpreting a tab keypress into an action message calls
[NSResponder insertTab]. [RenderWidgetHostViewCocoa
doCommandBySelector:] has for a long time had a check that skips
converting any action message whose selector starts with "insert" into
an editing command, and just treats it as a regular keypress instead.

This approach also works well for UI in MacViews, so do the same.

Theoretically a user could remap a different key or key combination to
the 'insertTab' action message, but that's unlikely.

Updates TextfieldTest.FocusTraversalTest with cross-platform checks to
test for this case. Also tested manually with views_examples.

For the test to work with the approach in this CL, the event simulator
needs to provide an answer for -[NSApplication currentEvent], so that's
done too.

BUG=454353

Review URL: https://codereview.chromium.org/951433002

Cr-Commit-Position: refs/heads/master@{#318172}