Use std::make_unique instead of absl::make_unique.

WebRTC is now using C++14 so there is no need to use the Abseil version
of std::make_unique.

This CL has been created with the following steps:

git grep -l absl::make_unique | sort | uniq > /tmp/make_unique.txt
git grep -l absl::WrapUnique | sort | uniq > /tmp/wrap_unique.txt
git grep -l "#include <memory>" | sort | uniq > /tmp/memory.txt

diff --new-line-format="" --unchanged-line-format="" \
  /tmp/make_unique.txt /tmp/wrap_unique.txt | sort | \
  uniq > /tmp/only_make_unique.txt
diff --new-line-format="" --unchanged-line-format="" \
  /tmp/only_make_unique.txt /tmp/memory.txt | \
  xargs grep -l "absl/memory" > /tmp/add-memory.txt

git grep -l "\babsl::make_unique\b" | \
  xargs sed -i "s/\babsl::make_unique\b/std::make_unique/g"

git checkout PRESUBMIT.py abseil-in-webrtc.md

cat /tmp/add-memory.txt | \
  xargs sed -i \
  's/#include "absl\/memory\/memory.h"/#include <memory>/g'
git cl format
# Manual fix order of the new inserted #include <memory>

cat /tmp/only_make_unique | xargs grep -l "#include <memory>" | \
  xargs sed -i '/#include "absl\/memory\/memory.h"/d'

git ls-files | grep BUILD.gn | \
  xargs sed -i '/\/\/third_party\/abseil-cpp\/absl\/memory/d'

python tools_webrtc/gn_check_autofix.py \
  -m tryserver.webrtc -b linux_rel

# Repead the gn_check_autofix step for other platforms

git ls-files | grep BUILD.gn | \
  xargs sed -i 's/absl\/memory:memory/absl\/memory/g'
git cl format

Bug: webrtc:10945
Change-Id: I3fe28ea80f4dd3ba3cf28effd151d5e1f19aff89
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153221
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Alessio Bazzica <alessiob@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29209}
diff --git a/rtc_base/openssl_certificate.cc b/rtc_base/openssl_certificate.cc
index c22e5fb..9459f76 100644
--- a/rtc_base/openssl_certificate.cc
+++ b/rtc_base/openssl_certificate.cc
@@ -20,7 +20,8 @@
 #include <openssl/pem.h>
 #include <time.h>
 
-#include "absl/memory/memory.h"
+#include <memory>
+
 #include "rtc_base/checks.h"
 #include "rtc_base/helpers.h"
 #include "rtc_base/logging.h"
@@ -138,7 +139,7 @@
 #if !defined(NDEBUG)
   PrintCert(x509);
 #endif
-  auto ret = absl::make_unique<OpenSSLCertificate>(x509);
+  auto ret = std::make_unique<OpenSSLCertificate>(x509);
   X509_free(x509);
   return ret;
 }
@@ -158,7 +159,7 @@
   if (!x509) {
     return nullptr;
   }
-  auto ret = absl::make_unique<OpenSSLCertificate>(x509);
+  auto ret = std::make_unique<OpenSSLCertificate>(x509);
   X509_free(x509);
   return ret;
 }
@@ -238,7 +239,7 @@
 }
 
 std::unique_ptr<SSLCertificate> OpenSSLCertificate::Clone() const {
-  return absl::make_unique<OpenSSLCertificate>(x509_);
+  return std::make_unique<OpenSSLCertificate>(x509_);
 }
 
 std::string OpenSSLCertificate::ToPEMString() const {