Hydrate when the page was server rendered (#29016)

Author: KyleAMathews

packages/gatsby/cache-dir/app.js

@@ -119,9 +119,11 @@ apiRunnerAsync(`onClientEntry`).then(() => {
  const renderer = apiRunner(
  `replaceHydrateFunction`,
  undefined,
- // TODO replace with hydrate once dev SSR is ready
- // but only for SSRed pages.
- ReactDOM.render
+ // Client only pages have any empty body so we just do a normal
+ // render to avoid React complaining about hydration mis-matches.
+ document.getElementById(`___gatsby`).children.length === 0
+ ? ReactDOM.render
+ : ReactDOM.hydrate
  )[0]
 
  let dismissLoadingIndicator