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cca8a0c0d709803ce3413861ccbdb1b47e8fdcef by Abseil Team <absl-team@google.com>:

Add AssertHeld() to SpinLock.

PiperOrigin-RevId: 418034654

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9e95a0e614b4cd7929d8db4324ca69d7be5351b2 by Evan Brown <ezb@google.com>:

Make btree use an unsigned size_type (size_t).

This brings btree in line with std::set, et al., which are required to use an unsigned size type - see https://en.cppreference.com/w/cpp/container/set.

Also avoid some warnings about comparing integers of different signs.

PiperOrigin-RevId: 418014466
GitOrigin-RevId: cca8a0c0d709803ce3413861ccbdb1b47e8fdcef
Change-Id: I2b951cf1d69a3bb9c8dc236c69ab4a06b45047ea