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This change removes DISALLOW_COPY_AND_ASSIGN from the
IllegalCharacters class, and makes member functions that don't modify
the class instance `const`.

Change-Id: I3b49140c4fb52aa6ae4982ed0bc6563dad6bd3a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2459637Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/master@{#828287}