Using thread_local on iOS requires iOS 9 or greater. Chrome for iOS
now requires 13, Skia sets the minimum to 11 for test builds, and
Flutter actively does not support 8 or earlier. Dropping support for
iOS 8 in practice and moving to iOS 9 makes it possible to use
thread_local without reservations on iOS.
Change-Id: Ib80cbe24e8154be650f343643281384c17356242
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447497
Reviewed-by: Heather Miller <hcm@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
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