Fix iframes affected by sandbox regression

A hotfix was pushed out which strips the sandbox
attribute from iframes. We should only strip it if the
iframe was affected by the regression introduced
with LS-743.

Test Plan:
Create a page
Add the following iFrame content:
<p><iframe src="https://docs.google.com/presentation/d/1X7DeC2XASzkjWsYX9eJuKid2Yxe_WxIksQP0mkYYl30/edit?usp=sharing" width="800" height="480" sandbox="allow-scripts allow-forms allow-same-origin"></iframe></p>
Save. On the page, try requesting access. A confirm
modal should appear.

Change the page content to the following:
<p><iframe src="https://docs.google.com/presentation/d/1X7DeC2XASzkjWsYX9eJuKid2Yxe_WxIksQP0mkYYl30/edit?usp=sharing" width="800" height="480" sandbox="allow-scripts allow-same-origin"></iframe></p>
Save. On the page, try requesting access. The confirm
modal should NOT appear and an error should be logged
to the console.

flag=none

Refs CNVS-49914

Change-Id: I20d0763a7ccf6e73163cf9056fdfad72efa2448c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/245002
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Alex Anderson <raanderson@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>

public/javascripts/instructure.js

@@ -241,7 +241,9 @@ export function enhanceUserContent() {
   }, 10)
   // Remove sandbox attribute from user content iframes to fix busted
   // third-party content, like Google Drive documents.
-  document.querySelectorAll('.user_content iframe[sandbox]').forEach(frame => {
+  document
+    .querySelectorAll('.user_content iframe[sandbox="allow-scripts allow-forms allow-same-origin"]')
+    .forEach(frame => {
       frame.removeAttribute('sandbox')
       const src = frame.src
       frame.src = src