Fix background of composition text

The Chromium API has been changed:
https://chromium-review.googlesource.com/c/chromium/src/+/959006/



Task-number: QTBUG-70289
Task-number: QTBUG-71453
Change-Id: Ib20a82bd826d8067aa3eb3bccccda7d8716e20ad
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>