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Historically official builds were always done exactly at mac_sdk_min,
to protect against official bots having new SDKs deployed on them before
the SDK roll was ready. However, this forces any upgrade of the official
bots to also change mac_sdk_min, which affects all other builds.

This change adds a separate mac_sdk_official_version gn variable, which
is the exact SDK version to use in official builds. This will allow us to
roll the SDK without forcing downstream users to upgrade to the 10.15 SDK
immediately.

Bug: 1023913
Change-Id: I7a9f3069456edabf51db1b0f91bf85b8a0907410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992195Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729828}