simplify ReceivedMessageFromTarget decoding
simplify ReceivedMessageFromTarget decoding

We implement our own UnmarshalEasyJSON, as we decode lots of these and
we want to avoid allocs. However, the message is never null, so we can
simplify the code.

chromedp/chromedp,chromedp/chromedp,knq/chromedp,chromedp/chromedp,knq/chromedp,knq/chromedp