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This change introduces base::Contains(), generalizing the existing
base::ContainsKey() and base::ContainsValue(). base::Contains() has
multiple templated overloads, depending on what API the queried
container provides. If possible, it invokes a contains() member function
on the passed in container. If this does not work, it will choose a
member find() function. If such a function is not present either, it will
use std::find() as a fallback.

Bug: 970209
Change-Id: I54fb619b6cf229308a40cc81658bd800c3671d40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1631406Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666228}