Use absl::make_unique and absl::WrapUnique directly

Instead of going through our wrappers in ptr_util.h.

This CL was generated by the following script:

  git grep -l ptr_util | xargs perl -pi -e 's,#include "rtc_base/ptr_util.h",#include "absl/memory/memory.h",'
  git grep -l MakeUnique | xargs perl -pi -e 's,\b(rtc::)?MakeUnique\b,absl::make_unique,g'
  git grep -l WrapUnique | xargs perl -pi -e 's,\b(rtc::)?WrapUnique\b,absl::WrapUnique,g'
  git checkout -- rtc_base/ptr_util{.h,_unittest.cc}
  git cl format

Followed by manually adding dependencies on
//third_party/abseil-cpp/absl/memory until `gn check` stopped
complaining.

Bug: webrtc:9473
Change-Id: I89ccd363f070479b8c431eb2c3d404a46eaacc1c
Reviewed-on: https://webrtc-review.googlesource.com/86600
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23850}

rtc_base/weak_ptr_unittest.cc

@@ -225,7 +225,7 @@ TEST(WeakPtrTest, WeakPtrInitiateAndUseOnDifferentThreads) {
   // Test that it is OK to create a WeakPtr on one thread, but use it on
   // another. This tests that we do not trip runtime checks that ensure that a
   // WeakPtr is not used by multiple threads.
-  auto target = rtc::MakeUnique<TargetWithFactory>();
+  auto target = absl::make_unique<TargetWithFactory>();
   // Create weak ptr on main thread
   WeakPtr<Target> weak_ptr = target->factory.GetWeakPtr();
   rtc::TaskQueue queue("queue");