[chromecast] Clear NSS client session cache in connectivity_checker
[chromecast] Clear NSS client session cache in connectivity_checker

Chromecast has no battery so its system time might be changing
during connectivity checking. Changing system time may be the
cause of cert error, such as DATE_INVALID, if negative result
is cached.

Clear client session cache before checking or when cert error
is encountered.

BUG=internal b/23619920

Review URL: https://codereview.chromium.org/1315523009

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#346540}

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