Chrome Version       : 56.0.2906.0
OS Version: OS X 10.12.1

AppKit provides limited methods for creating dummy NSEvent objects for testing. The third mouse button and trackpad scrolling events are particularly problematic.

cocoa_test_event_utils::TestScrollEvent is evolving support for trackpad scroll event sequences. Currently used in:
 events_mac_unittest -- EventsMacTest::TrackpadScrollSequence(), and
 ui/views/test/event_generator_delegate_mac - used to make mousewheel scroll events


Some limitations of the current setup:
 - Only event_generator_delegate_mac::MouseEventAtPoint has support for a middle click. (MouseEventAtPointInWindow will silently fail to create middle-click events -- these should use a common codepath; possibly:
 - EventsMacTest::TestMouseEvent(), which creates events with both -[NSEvent window] and middle clicks, but it gets dispatched differently :/ -- see https://codereview.chromium.org/2484863004/
 - A Mighty Mouse makes a special scroll sequence that we don't have tests for
 - Making test NSEvents with +[NSEvent eventWithCGEvent:] requires a special dance, e.g., for coordinate conversion
  - See https://codereview.chromium.org/2486643002/
 - cocoa_test_event_utils.mm's `AttachWindowToCGEvent` uses KVO to set the -[NSEvent window] argument -- it "works" but it doesn't feel right. E.g. the events get dispatched differently if sending to NSApp (but they seem to go fine if they're dispatched directly to NSWindow).