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Verify that the certificate presented by the alternative server is valid for the
origin when using HTTP/2, both when opening a new connection or when pooling to
an existing one.

Patch Set 1:
 * Enable hopping to a different host for HTTP/2 (not for QUIC).
 * Add unittests for both new connection and pooling cases.  Note in trybot
   output that *Valid tests pass and *Invalid tests fail.

Patch Set 2 and up:
 * Verify certificate validity in HttpStreamFactoryImpl::Job.

BUG=474217

Review URL: https://codereview.chromium.org/1074193003

Cr-Commit-Position: refs/heads/master@{#327293}