Support forward delete menu item accelerator

[kevinsawicki]

Not sure if/when/how this might have regressed, but using an accelerator of Delete no longer works as reported in #5419.

This looks to be because ui::MacKeyCodeForWindowsKeyCode from Chrome is returning NSDeleteFunctionKey as the character instead of NSDeleteCharacter.

From the NSMenuItem keyEquivalent docs:

If you want to specify the Backspace key as the key equivalent for a menu item, use a single character string with NSBackspaceCharacter (defined in NSText.h as 0x08) and for the Forward Delete key, use NSDeleteCharacter (defined in NSText.h as 0x7F). Note that these are not the same characters you get from an NSEvent key-down event when pressing those keys.

https://cs.chromium.org/chromium/src/ui/events/keycodes/keyboard_code_conversion_mac.mm?l=75

This pull request adds a check and mapping for this case which gets Delete working again as an accelerator for a menu like:

{
  label: 'Backspace',
  accelerator: 'Backspace'
},
{
  label: 'Delete',
  accelerator: 'Delete'
}

Before

### After

Closes #5419

[zcbenz]

👍