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This change inlines the defaulted assignment operators in flat_tree.
Furthermore, this works around a compiler bug exposed in old GCC
versions.

Fixed: 1154608
Change-Id: I488b6003de6630d2d18b7849bc903db2d41ed9bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566943
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833080}