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This CL removes an unused header, histogram_functions.

Bug: None
Change-Id: I1b96ab271f0b08984eedb400c31514767de0ebf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2395736Reviewed-by: Richard Knoll <knollr@chromium.org>
Commit-Queue: Mohamed Adel <adelm@google.com>
Cr-Commit-Position: refs/heads/master@{#804716}