storage: inline FileSystemAccessFileHandleImpl::GetSwapURL

GetSwapURL (only called once, not counting its dedicated test) became
trivial after crrev.com/c/4574813 "storage: add
FileSystemURL::CreateSibling method" landed.

The FileSystemAccessFileHandleImplTest.GetSwapURL test, testing that
buckets are preserved by GetSwapURL/CreateSibling, is redundant after
the FileSystemURLTest.CreateSiblingPreservesBuckets test was added.

BUG=None
TEST=CQ

Change-Id: Id02c841e7a40a0405ecfb95e475b97c01b419fc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4617793


Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1159584}