blink: Allow use of base::void_t in //third_party/blink/

base::void_t is the same feature as std::void_t, which is supported
in C++17.  Chromium currently follows C++14 and base::void_t is an
exceptional support of a C++17 feature.

void_t is a simple mapping from arbitrary type(s) to type void and
nothing is harmful.
https://en.cppreference.com/w/cpp/types/void_t

This patch adds base::void_t to the allowed list of Blink.

FYI, Blink somehow already uses base::void_t at two places:
//third_party/blink/renderer/platform/heap/finalizer_traits.h
//third_party/blink/renderer/platform/wtf/type_traits.h

Change-Id: I1e1f0447027df58c85b6a8f20db10df4aac2796b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2002552Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732012}

third_party/blink/renderer/DEPS

@@ -36,6 +36,7 @@ include_rules = [
     "+base/sys_byteorder.h",
     "+base/system/sys_info.h",
     "+base/task/post_task.h",
+    "+base/template_util.h",
     "+base/test/metrics/histogram_tester.h",
     "+base/test/mock_callback.h",
     "+base/test/scoped_feature_list.h",

third_party/blink/tools/blinkpy/presubmit/audit_non_blink_usage.py

@@ -169,6 +169,9 @@ _CONFIG = [
             'base::ClampSub',
             'base::MakeClampedNum',
 
+            # //base/template_util.h.
+            'base::void_t',
+
             # Debugging helpers from //base/debug are allowed everywhere.
             'base::debug::.+',