[Contextual Tasks] Fix isFrameLoading never false

I must've missed changing isFrameLoading back to false when I refactored
the code. Adding it now with tests to catch in the future.

Change-Id: I59b67a179fb0077fa9b730e2c7449648e1d6c6da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7594568
Commit-Queue: Duncan Mercer <mercerd@google.com>
Reviewed-by: Bryan Nguyen <nguyenbryan@google.com>
Cr-Commit-Position: refs/heads/main@{#1588967}