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This change modifies base's implementation of C++20's uniform container
erasure API to reflect the latest changes from the committee meeting in
Prague. In particular, these functions now return the number of erased
elements, rather than having no return value. Differences from the
standard [1]:

- For simplicity, base::Erase and base::EraseIf return size_t, rather
  than typename Container::size_type.
- Since std::forward_list::remove_if only returns size_type starting
  from C++20, the respective overloads need to perform two extra O(n)
  operations to calculate the number of removed elements.

[1] https://wg21.link/p1115

Bug: 697235
Change-Id: I30ad6ce08f3a37eddd1483e4fdc4f1cd8d155d2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2072038Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744996}