browser: provide a default filename when unable to generate from download data

[deepak1556]

Also we didn't handle alt+click on links with no download attribute, chrome downloads for that gesture but we generate a new-window event. This patch fixes it in a separate commit, can be reverted if not desired.

Fixes #7151

[zcbenz]

This changes current behavior and not all users want to have the ability download things from the app, we can probably just make it explicit in the new-window event and document it.

[deepak1556]

Fixed, thanks!

[kevinsawicki]

Looks like the spec introduced here is consistently failing on AppVeyor with the following error:

not ok 253 session module DownloadItem can generate a default filename
  AssertionError: 'download' == 'download.pdf'
      at EventEmitter.<anonymous> (C:\projects\electron\spec\api-session-spec.js:333:18)
      at EventEmitter.g (events.js:286:16)
      at emitMany (events.js:127:13)
      at EventEmitter.emit (events.js:201:7)

It doesn't appear to be failing though on our internal Windows build machines which is really strange.