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This patch
* Mutates `currentFiles` upon deletion and tries to open the next file.
* allows empty fileLists in `advance()` and `sendFilestoGuest()`
* Adds a test for the scenario of deleting files in a directory
of images.
* reworks some of DeleteOriginalIPC to make the test more
robust as before it was passing due to FakeFileSystemHandle always having
the same default file name. Explicitly adding a filename makes the test
more obvious when debugging.
* Also swap DeleteOriginalIPC assert calls around to have the expected
value on the LHS as we use the `assertEquals` from
go/crcs/chrome/test/data/webui/test_api.js

Bug: 1077732, b/156042029
Change-Id: I1606809db9ccae388182a086aa9045280624c5dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206195
Commit-Queue: David Lei <dlei@google.com>
Reviewed-by: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770928}