Have way to specify unknown size in the spec

[ryanthompson591]

Sometimes on initialization of a preroll the size of the video might not yet be known.

In this case it would be useful in the spec to have a way to let the ad know that the dimensions are not yet known.

Proposal:
Dimensions may use -1 to specify when something is unknown.

For example

videoDimensions: {
x: 0,
y: 0,
width: -1,
height: -1,
}

would indicate that the video will be in the top left corner but the width and height are not yet known.

[mimidden]

Committee accepts this.