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Signature verification of signed exchange takes some time (>10ms on
Nexus 5X), so total latency of SignedExchange loading can be reduced by
starting certificate verification before doing signature verification.

Unit tests are changed so that mock verifiers don't complete Verify()
synchronously.

Change-Id: Idc8e222c810e3a65db9072b21ed852daf8bcd316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309517Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794373}