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This adds WARN_UNUSED_RESULT to some methods in //base/strings
whose primary purpose is to return something useful (e.g. StrCat).
When the results of these methods go unused, it's usually caller error
and will be found by the caller later (for instance, when their tests
fail). WARN_UNUSED_RESULT makes this fail earlier, at compile time.

Precedent: Abseil's StrCat, etc., all use ABSL_MUST_USE_RESULT.
Change-Id: I8262b03bd844118c7337861f323417c2bb92ddda
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925251Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: David Van Cleve <davidvc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717633}