Allow base/metrics/histogram.h to be used directly in Blink.

It's already used in a number of places in Blink. Blessing direct use of
the standard UMA histogram macros will also make it simpler to make
improvements to things like the macros for enumerations.

Bug: 742517
Change-Id: Ic7b83312e77918170c0dbabc92c1536fed673abc
Reviewed-on: https://chromium-review.googlesource.com/982946Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546744}

third_party/WebKit/Source/DEPS

@@ -8,6 +8,7 @@ include_rules = [
     "+base/macros.h",
     "+base/memory/ptr_util.h",
     "+base/memory/weak_ptr.h",
+    "+base/metrics/histogram_macros.h",
     "+base/sequenced_task_runner.h",
     "+base/single_thread_task_runner.h",
     "+base/task_scheduler/post_task.h",