[Cleanup] Update histogram_tester.h includes in //content

Update all histogram_tester.h includes in //content to include
//base/test/metrics/histogram_tester.h

[email protected]

Bug: 846421
Change-Id: Id1f2d111e1b71e32ca795a7953af63fb62ad2acd
Reviewed-on: https://chromium-review.googlesource.com/1086269
Reviewed-by: Devlin <[email protected]>
Commit-Queue: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#564514}
38 files changed