Fix #955: Use robots.jpeg from html.spec instead of images.whatwg.org

We cannot use a relative URL like /images/robots.jpeg due to something
about iframe data: URLs as srcs.

PR: #961

source

@@ -36044,7 +36044,7 @@ The General Relativistic Field Equations</pre>
   score. Suppose a robotics competition was being streamed live. The image could be overlayed with
   the scores, as follows:</p>
 
-  <p><iframe src="data:text/html;charset=utf-8,%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20body%2C%20html%20%7B%20margin%3A%200%3B%20padding%3A%200%3B%20overflow%3A%20hidden%3B%20%7D%0A%20div%20%7B%20width%3A%20600px%3B%20height%3A%20400px%3B%20position%3A%20relative%3B%20%7D%0A%20p%20%7B%20position%3A%20absolute%3B%20top%3A%200%3B%20margin%3A%200.25em%3B%20font%3A%20small-caps%20900%202em%20sans-serif%3B%20text-shadow%3A%20white%200%200%204px%3B%20%7D%0A%20span%20%7B%20display%3A%20block%3B%20%7D%0A%20.left%20%7B%20color%3A%20red%3B%20left%3A%200%3B%20text-align%3A%20left%3B%20%7D%0A%20.right%20%7B%20color%3A%20blue%3B%20right%3A%200%3B%20text-align%3A%20right%3B%20%7D%0A%20.middle%20%7B%20color%3A%20white%3B%20top%3A%20auto%3B%20bottom%3A%200%3B%20left%3A%200%3B%20right%3A%200%3B%20text-align%3A%20center%3B%20text-shadow%3A%20black%200%200%204px%3B%20%7D%0A%20.middle%20span%20%7B%20display%3A%20inline-block%3B%20margin%3A%200%201em%3B%20font-size%3A%200.75em%3B%20text-transform%3A%20uppercase%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%3Cimg%20src%3D%22https%3A%2F%2Fimages.whatwg.org%2Frobots.jpeg%22%3E%0A%20%3Cp%20class%3D%22score%20left%22%3E%3Cspan%3ERed%20Alliance%3C%2Fspan%3E%20%3Cspan%3E78%3C%2Fspan%3E%3C%2Fp%3E%0A%20%3Cp%20class%3D%22score%20right%22%3E%3Cspan%3EBlue%20Alliance%3C%2Fspan%3E%20%3Cspan%3E66%3C%2Fspan%3E%3C%2Fp%3E%0A%20%3Cp%20class%3D%22score%20middle%22%3E%3Cspan%3EQual%20Match%2037%3C%2Fspan%3E%20%3Cspan%3EFriday%2014%3A21%3C%2Fspan%3E%0A%3C%2Fdiv%3E" width="600" height="400"></iframe>
+  <p><iframe src="data:text/html;charset=utf-8,%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20body%2C%20html%20%7B%20margin%3A%200%3B%20padding%3A%200%3B%20overflow%3A%20hidden%3B%20%7D%0A%20div%20%7B%20width%3A%20600px%3B%20height%3A%20400px%3B%20position%3A%20relative%3B%20%7D%0A%20p%20%7B%20position%3A%20absolute%3B%20top%3A%200%3B%20margin%3A%200.25em%3B%20font%3A%20small-caps%20900%202em%20sans-serif%3B%20text-shadow%3A%20white%200%200%204px%3B%20%7D%0A%20span%20%7B%20display%3A%20block%3B%20%7D%0A%20.left%20%7B%20color%3A%20red%3B%20left%3A%200%3B%20text-align%3A%20left%3B%20%7D%0A%20.right%20%7B%20color%3A%20blue%3B%20right%3A%200%3B%20text-align%3A%20right%3B%20%7D%0A%20.middle%20%7B%20color%3A%20white%3B%20top%3A%20auto%3B%20bottom%3A%200%3B%20left%3A%200%3B%20right%3A%200%3B%20text-align%3A%20center%3B%20text-shadow%3A%20black%200%200%204px%3B%20%7D%0A%20.middle%20span%20%7B%20display%3A%20inline-block%3B%20margin%3A%200%201em%3B%20font-size%3A%200.75em%3B%20text-transform%3A%20uppercase%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%3Cimg%20src%3D%22https%3A%2F%2Fhtml.spec.whatwg.org%2Fimages%2Frobots.jpeg%22%3E%0A%20%3Cp%20class%3D%22score%20left%22%3E%3Cspan%3ERed%20Alliance%3C%2Fspan%3E%20%3Cspan%3E78%3C%2Fspan%3E%3C%2Fp%3E%0A%20%3Cp%20class%3D%22score%20right%22%3E%3Cspan%3EBlue%20Alliance%3C%2Fspan%3E%20%3Cspan%3E66%3C%2Fspan%3E%3C%2Fp%3E%0A%20%3Cp%20class%3D%22score%20middle%22%3E%3Cspan%3EQual%20Match%2037%3C%2Fspan%3E%20%3Cspan%3EFriday%2014%3A21%3C%2Fspan%3E%0A%3C%2Fdiv%3E" width="600" height="400"></iframe>
 
   <p>In order to make the score display render correctly whenever the user seeks to an arbitrary
   point in the video, the metadata text track cues need to be as long as is appropriate for the