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PushMessagingAppIdentifier::FindByAppId was DCHECKing when called from
PushMessagingServiceImpl::CanHandle to see if an app_id belongs to
Push, since it assumes that it would only be called with valid Push
app_ids.

This patch makes it test whether the app_id is a Push Messaging app_id
before performing any validation DCHECKs.

BUG=489304

Review URL: https://codereview.chromium.org/1140873005

Cr-Commit-Position: refs/heads/master@{#330378}