Make full-page-screenshot resistant to layout shifts

[connorjclark]

ref #10716 (comment)

Possible fixes:

• somehow iterate all the elements we gathered w/ bounding rects and do a last-call on their position
• there's more in that comment but I don't think they are a full solution

wip branch: resolve-nodes

[paulirish]

Possible fixes:
[...]
there's more in that comment but I don't think they are a full solution

hah yeah from the comment I mentioned the 3 below items

1. disabling JS
   • we've since looked into this. it almost seems to work. we do a await driver.sendCommand('Emulation.setScriptExecutionDisabled', {value: true}); on either side of GatherRunner.afterPass.. Setting this means the page cannot run JS however our evaluateAsync is immune to that.. However a setTimeout within evaluateAsync will be a problem and unfortunately axe has some (though they're only for splitting up long tasks. lol).
   • we could work around that. but would we still have perfect positions? it's not guaranteed, though mostly edge cases.
2. keeping the full-size screenshot viewport applied while the elem dimensions are being collected.
   • this is what connor is currently investigating.
3. The "pause page activity" effort from dgozman is also relevant.
   • relevant but on hold. no news there.