dpwas: Update protocol check to validate against specific webapp

web_app_start_utils.cc doesn't need to check for all apps, only the
app specified via the command line and deletes some dead code.

With the dead code removed, ProtocolHandlerManagerTest is no longer
needed, which then left WebAppProtocolHandlerManager with zero unit
test coverage. This change also adds new unit test coverage for
WebAppProtocolHandlerManager.

Repro steps:

1) Install https://mwjacksonmsft.github.io/pwa/display-standalone/
2) Run chrome_proxy.exe --app-id=cppdpmhljndhkpjhhflifimgckghnmak web+testing://test
   This should prompt the user for a protocol handler launch.
3) Run chrome_proxy.exe --app-id=cppdpmhljndhkpjhhflifimgckghnmak web+test://test
   This should not prompt, and load the app at the default entry point.

Bug: 1249907
Change-Id: Ia155db2e30e4c1fd12628714d33c49dc3155e8f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3617362
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Mike Jackson <mjackson@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#998430}