[3PWebAPIRequestFwd] ThirdPartyCredentialManagerImpl improvements

The handling of get requests by `ThirdPartyCredentialManagerImpl` got
improved with the following fixes:
- Handle SSL errors by returning an empty credential.
- Handle incognito mode by returning an empty credential.
- Handle silent and conditional mediation by returning an empty list of
credentials (but treat the request as successful because no errors
occurred).

Change-Id: I3ccc8e59e589ba8f33f79998313cffcc4611e6fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6652690
Reviewed-by: Friedrich Hauser <friedrichh@chromium.org>
Commit-Queue: Ivana Žužić <izuzic@google.com>
Cr-Commit-Position: refs/heads/main@{#1476422}