bug(android): DOMRect polyfill needed for early ES6 devices

[sentry-io]

Sentry Issue: KEYMAN-WEB-JD

Error:  Error from queued closure: DOMRect is not defined

ReferenceError: DOMRect is not defined
    at se.getBoundingClientRect (file:///data/user/0/com.tavultesoft.kmapro/app_data/keymanweb-webview.js:15:6053)
    at itemIdentifier (file:///data/user/0/c...
  at Nr.<anonymous> (@keymanapp/keyman/common/web/gesture-recognizer/src/engine/headless/asyncClosureDispatchQueue.ts:60:7)
  at Generator.next (<anonymous>)
  at Promise (<anonymous>)
  at AsyncClosureDispatchQueue.triggerNextClosure (@keymanapp/keyman/common/web/gesture-recognizer/src/engine/headless/asyncClosureDispatchQueue.ts:39:38)
  at Nr.<anonymous> (@keymanapp/keyman/common/web/gesture-recognizer/src/engine/headless/asyncClosureDispatchQueue.ts:83:10)
...
(23 additional frame(s) were not displayed)

Per MDN, DOMRect requires Chrome 61, which is later than full ES6 support was added. Despite this, the ES6 version of the keyboard host-page does not include the DOMRect polyfill that the ES5 version does.

[jahorton]

Reopening due to breakage occurring from merge of #11881, which unlinked this polyfill despite not raising the minimum requirements enough to mitigate the need for it.