renderer: fix desktop capture api not responding different subsequest calls

[deepak1556]

Race condition between media_list reset on DesktopCapturer::StartHandling and DesktopCapturer::OnRefreshFinished

Fixes #5269

[zcbenz]

I'm not sure if it is the root cause, but the test doesn't pass in CI:

not ok 79 desktopCapturer responds to subsequest calls of different options
  AssertionError: 0 != 0
      at callback (/home/travis/build/electron/electron/spec/api-desktop-capturer-spec.js:32:14)
      at EventEmitter.<anonymous> (/home/travis/build/electron/electron/out/D/resources/electron.asar/renderer/api/desktop-capturer.js:33:12)
      at EventEmitter.g (events.js:273:16)
      at emitTwo (events.js:100:13)
      at EventEmitter.emit (events.js:185:7)

[deepak1556]

have fixed the failing test, the screen capture sources was returning empty on the CI.

[zcbenz]

👍