Revert of Delete unused code from systeminfo. (patchset #3 id:40001 of https://codereview.webrtc.org/2578323005/ )

Reason for revert:
Breaks downstream build.

Original issue's description:
> Delete unused code from systeminfo.
>
> BUG=webrtc:6906
>
> Review-Url: https://codereview.webrtc.org/2578323005
> Cr-Commit-Position: refs/heads/master@{#15655}
> Committed: https://chromium.googlesource.com/external/webrtc/+/617ca316e9363986a54926a185fac799de3869c2

TBR=perkj@webrtc.org,kthelgason@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6906

Review-Url: https://codereview.webrtc.org/2584563004 .
Cr-Commit-Position: refs/heads/master@{#15660}
diff --git a/webrtc/system_wrappers/source/cpu_info.cc b/webrtc/system_wrappers/source/cpu_info.cc
index 4a61c7a..40231b6 100644
--- a/webrtc/system_wrappers/source/cpu_info.cc
+++ b/webrtc/system_wrappers/source/cpu_info.cc
@@ -10,60 +10,12 @@
 
 #include "webrtc/system_wrappers/include/cpu_info.h"
 
-#if defined(WEBRTC_WIN)
-#include <winsock2.h>
-#include <windows.h>
-#ifndef EXCLUDE_D3D9
-#include <d3d9.h>
-#endif
-#elif defined(WEBRTC_LINUX)
-#include <unistd.h>
-#endif
-#if defined(WEBRTC_MAC)
-#include <sys/sysctl.h>
-#endif
-
-#include "webrtc/base/logging.h"
-
-namespace internal {
-static int DetectNumberOfCores() {
-  // We fall back on assuming a single core in case of errors.
-  int number_of_cores = 1;
-
-#if defined(WEBRTC_WIN)
-  SYSTEM_INFO si;
-  GetSystemInfo(&si);
-  number_of_cores = static_cast<int>(si.dwNumberOfProcessors);
-#elif defined(WEBRTC_LINUX) || defined(WEBRTC_ANDROID)
-  number_of_cores = static_cast<int>(sysconf(_SC_NPROCESSORS_ONLN));
-#elif defined(WEBRTC_MAC)
-  int name[] = {CTL_HW, HW_AVAILCPU};
-  size_t size = sizeof(number_of_cores);
-  if (0 != sysctl(name, 2, &number_of_cores, &size, NULL, 0)) {
-    LOG(LS_ERROR) << "Failed to get number of cores";
-    number_of_cores = 1;
-  }
-#else
-  LOG(LS_ERROR) << "No function to get number of cores";
-#endif
-
-  LOG(LS_INFO) << "Available number of cores: " << number_of_cores;
-
-  return number_of_cores;
-}
-}
+#include "webrtc/base/systeminfo.h"
 
 namespace webrtc {
 
 uint32_t CpuInfo::DetectNumberOfCores() {
-  // Statically cache the number of system cores available since if the process
-  // is running in a sandbox, we may only be able to read the value once (before
-  // the sandbox is initialized) and not thereafter.
-  // For more information see crbug.com/176522.
-  static uint32_t logical_cpus = 0;
-  if (!logical_cpus)
-    logical_cpus = static_cast<uint32_t>(internal::DetectNumberOfCores());
-  return logical_cpus;
+  return static_cast<uint32_t>(rtc::SystemInfo::GetMaxCpus());
 }
 
 }  // namespace webrtc