Trigger Docs html fallback with script added to document head

Do this to address possible issues with document.body availability.

R=katie@chromium.org

Test: manual.
Change-Id: I42a727829a8552de1609bdc8b3af2f10503c4121
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640730Reviewed-by: Katie Dektar <katie@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845464}

chrome/browser/resources/chromeos/accessibility/common/gdocs_script.js

@@ -10,7 +10,7 @@ function triggerDocsHtmlFallbackMode() {
   `;
   const script = document.createElement('script');
   script.innerHTML = scriptContents;
-  document.body.appendChild(script);
+  document.head.appendChild(script);
 }
 
 function RemoveAriaHiddenFromGoogleDocsContent() {