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For non-official builds, typically Widevine signing certs are not
available. This CL updates `enable_widevine_cdm_host_verification` so
it's only set to true by default when `is_official_build` is true.

For developers enabling `is_official_build` and `is_chrome_branded`
locally without Widevine signing certs, the build may fail. In this case
simply set `enable_widevine_cdm_host_verification` to false to suppress
the error.

Bug: 1127616
Change-Id: I4256f8c53d79907eec23782300e223b4edb89d75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410311Reviewed-by: John Rummell <jrummell@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806802}