Add a Send to Device subview to the page action panel. When the page isn't
sendable, disable the Send to Device menu item. When the user doesn't have any
devices, show a menu item that opens the Firefox Account preferences pane.
Generalize gSync.populateSendTabToDevicesMenu() so that it can be used to
populate any kind of container, not only a menupopup with menuitems.
Add an SVG that shows a phone and an SVG that shows a desktop.
MozReview-Commit-ID: EZQKAEAr08q
This is unfortunate, but in order to keep automated tests working when we flip the pref,
it would be nice to be able to flip the pref at runtime and have things Just Work. This
tries to make that happen. We can remove most of this code post-Photon.
MozReview-Commit-ID: 8zbgCGJautO
