Focus the iframes when moving through shot creation flow

[jaredhirsch]

Tab focus is lost when moving from preselection to selection iframe in the shot creation flow (from step 1 to step 2).

We should put the focus on one of the elements in the overlay when transitioning between frames.

[punamdahiya]

@6a68 looks like this one should be handled in web extension as well.

[chenba]

Not sure I quite understand the description. Nothing is auto focused on the preselection frame, so I don't know what's lost or different when moving to the selection frame. The one difference is that the buttons on the preselection frame have tabindexes whereas those on the selection frame do not. Tabbing through the buttons work on both frames.