[FilesBulkPinning] Fix up tests failing

The changes introduced in CL:4654105 caused the settings page to
periodically get the offline space usage. On slower bots this can cause
the function to fire twice, which causes the WillOnce clauses to fail.
Introduce the WillRepeatedly to ensure the slower bots get data
returned.

Fixed: 1459504
Test: browser_tests --gtest_filter=*GoogleDriveHandler*
Change-Id: Ic8d146c1cbfbe1c4ef85f5b633b91121cbac2a17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4659959
Auto-Submit: Ben Reich <benreich@chromium.org>
Reviewed-by: François Degros <fdegros@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1165158}