Show a default image on the virtual control interface (SMTC/MPRIS)

The image specified by GetDefaultFaviconURL cannot be shown on the virtual control interface.

We should probably set the Firefox's logo to default metadata image on the virtual control interface, if media-session doesn't set it. However, it seems the path of the Firefox logo is different on every platform. I wonder if there is a way to get the path.

In bug 1623971, for the default image(Firefox logo) that is trustworthy, we can feed the link to the underlying platform API directly without using FetchImageHelper since the path of default image starts with "file://". But we need to kwow the path of the logo first.

Here are some references

• Linux part (MPRIS):
  1. https://searchfox.org/mozilla-central/rev/7cadba1d8b8feaec4615f5bb98aac4b8a719793c/widget/gtk/nsWindow.cpp#2375
  2. https://searchfox.org/mozilla-central/rev/7cadba1d8b8feaec4615f5bb98aac4b8a719793c/widget/nsBaseWidget.cpp#1553
• Windows: https://searchfox.org/mozilla-central/rev/7cadba1d8b8feaec4615f5bb98aac4b8a719793c/widget/windows/nsWindow.cpp#3670

Here should list what image files we have:

• Windows: https://searchfox.org/mozilla-central/rev/7cadba1d8b8feaec4615f5bb98aac4b8a719793c/browser/installer/package-manifest.in#133-134
• GTK: https://searchfox.org/mozilla-central/rev/7cadba1d8b8feaec4615f5bb98aac4b8a719793c/browser/installer/package-manifest.in#256-260

Chrome doesn't show default image on the SMTC(Windows) or MPRIS(Linux) interface if the media-session doesn't set it. If the image starts with "file://", Chrome doesn't show it as well.