Min-Vid player is closed when pressing "Close" button from Nightly if browser.tabs.warnOnClose pref is set to true

[Softvision-VladBacia]

[Affected versions]: -Nightly 54.0a1 -Min-Vid 0.3.4-dev (built on 02/21/2017)

[Affected Platforms]: -All Windows -All Mac OS X -All Linux

[Prerequisites]: -Latest Min-Vid add-on version (0.3.3b) was previously installed on a clean profile. -In about:preferences the "When Nightly starts" option is set to any other than Show your windows and tabs from last time. -In about:config the browser.tabs.warnOnClose pref is set to true .

[Steps to reproduce]:

1. Open the browser with the profile from prerequisites (a fresh restart is required).
2. Open multiple websites in Nightly tabs.
3. Navigate to https://www.youtube.com and send a video to the mini player.
4. Click on "X" (Close) button from the Nightly window.

[Expected result]:

• Min-Vid player is not closed. Min-Vid player should be closed only after the user has confirmed the close action by clicking the "Close tabs" from the Confirm Close dialog. If "Cancel" is selected

[Actual result]:

• The Min-Vid player is closed.

[Additional Notes]:

• After the Min-Vid player has closed, if another video is sent to it then the scenario from #539 is reproducible.