documentScan: Simplify scanner id token tracking

The backend now generates per-client anonymized tokens for scanners
instead of returning the SANE connection.  This allows the backend to
make use of its more detailed knowledge of device availability to change
the tokens when needed instead of on every GetScannerList call.

Since these tokens are already anonymous, DocumentScanAPIHandler no
longer needs to replace them with its own tokens.  Instead, remove all
that tracking logic and just track the set of active scanner IDs for
each extension.  Now that the handler is able to clear scanner IDs per
extension instead of globally, it also makes sense to clear the lists of
scanner handles and job handles at the same time to avoid having them
grow indefinitely.

Finally, this also removes the shared scanner_devices map in favor of
tracking directly inside ExtensionState.  The keys provided by the
backend are no longer shared across extensions, so each extension is
effectively only accessing a disjoint slice of the shared map anyway.

Bug: b/316152239
Bug: b/315310310
Test: Scan with test extension
Change-Id: Ic0351e42d3b087369068c9833efe10063120ab09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5241243
Reviewed-by: Piotr Pawliczek <pawliczek@chromium.org>
Commit-Queue: Benjamin Gordon <bmgordon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1253721}

• chrome/browser/extensions/api/document_scan/document_scan_api_handler.cc[diff]
• chrome/browser/extensions/api/document_scan/document_scan_api_handler.h[diff]
• chrome/browser/extensions/api/document_scan/document_scan_api_handler_unittest.cc[diff]
• chrome/browser/extensions/api/document_scan/document_scan_test_utils.cc[diff]
• chrome/test/data/extensions/api_test/document_scan/perform_scan.js[diff]