v2.0.0

Update the shape of the ResizeObserverEntry to match the latest version of the spec.

entry = {
  target: <element>,
  contentRect: { /* v1 backwards compat */ },
  borderBoxSize: { inlineSize, blockSize },
  contentBoxSize: { inlineSize, blockSize }
}

• Removes support for scroll-box.
• Removes support for device-pixel-border-box. This will be added when/if defined in the spec.
• Updates contentSize to contentBoxSize.