QA: Fix "Cancelling reply preview" browser test

Now that bug 69412 is fixed, the test can work.
The preview warning CSS changed, and there's a Keep editing step.
Should work in all browsers, so adjust the annotations.

Change-Id: I857989dc11e9dac4e2fec534989afc8807e57deb
Merge "QA: Fix "Cancelling reply preview" browser test"