Mac: Create undefined gesture events for begin and end NSEvents
Mac: Create undefined gesture events for begin and end NSEvents

A single gesture in Cocoa may contain multiple sub-gestures (e.g, both
pinch and rotate). The begin/end-gesture brackets that we get from Cocoa
do not specify which kind of sub-gesture they are for.

Allow the event factory to create Undefined-type events for these
Cocoa events. The callers can make them more specific later.

BUG=451559

Review URL: https://codereview.chromium.org/865393011

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@189292 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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