Removed unused header

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}

content/browser/notifications/platform_notification_context_unittest.cc

@@ -8,7 +8,6 @@
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/memory/ptr_util.h"
-#include "base/metrics/histogram_functions.h"
 #include "base/run_loop.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/bind_test_util.h"