Delegate WIN32_LEAN_AND_MEAN definition to the toolchain.

The macro WIN32_LEAN_AND_MEAN is always defined by the toolchain, see
the config //build/config/win:lean_and_mean [1].

This means that WebRTC should not define it in its source files, this
causes two problems:

1. rtc_base:rtc_base_generinc needs to use /wd4005 to avoid a compiler
   warning (because the macro is redefined without checking).
2. the macro is not consistently defined before including windows.h,
   which means that WebRTC already relies on the toolchain almost
   everywhere.

[1] - https://cs.chromium.org/chromium/src/build/config/win/BUILD.gn?l=514&rcl=54f7be95955c9b35e64569c1d40e0c3dd097359c

Bug: webrtc:9251
Change-Id: I2b0b3c16856023db2a38a38f7652bb849ff283c7
Reviewed-on: https://webrtc-review.googlesource.com/87582
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23889}

rtc_base/BUILD.gn

@@ -950,11 +950,6 @@ rtc_static_library("rtc_base_generic") {
       "secur32.lib",
     ]
 
-    cflags += [
-      # Suppress warnings about WIN32_LEAN_AND_MEAN.
-      "/wd4005",
-    ]
-
     defines += [ "_CRT_NONSTDC_NO_DEPRECATE" ]
   }
 

rtc_base/logging.cc

@@ -9,9 +9,6 @@
  */
 
 #if defined(WEBRTC_WIN)
-#if !defined(WIN32_LEAN_AND_MEAN)
-#define WIN32_LEAN_AND_MEAN
-#endif
 #include <windows.h>
 #if _MSC_VER < 1900
 #define snprintf _snprintf

rtc_base/physicalsocketserver.cc

@@ -32,7 +32,6 @@
 #endif
 
 #if defined(WEBRTC_WIN)
-#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include <winsock2.h>
 #include <ws2tcpip.h>

rtc_base/sigslot.h

@@ -110,9 +110,6 @@
 #define _SIGSLOT_SINGLE_THREADED
 #elif defined(WEBRTC_WIN)
 #define _SIGSLOT_HAS_WIN32_THREADS
-#if !defined(WIN32_LEAN_AND_MEAN)
-#define WIN32_LEAN_AND_MEAN
-#endif
 #include "rtc_base/win32.h"
 #elif defined(__GNUG__) || defined(SIGSLOT_USE_POSIX_THREADS)
 #define _SIGSLOT_HAS_POSIX_THREADS

rtc_base/timeutils.cc

@@ -18,9 +18,6 @@
 #endif
 
 #if defined(WEBRTC_WIN)
-#ifndef WIN32_LEAN_AND_MEAN
-#define WIN32_LEAN_AND_MEAN
-#endif
 // clang-format off
 // clang formatting would put <windows.h> last,
 // which leads to compilation failure.

rtc_base/win32.h

@@ -15,10 +15,6 @@
 #error "Only #include this header in Windows builds"
 #endif
 
-#ifndef WIN32_LEAN_AND_MEAN
-#define WIN32_LEAN_AND_MEAN
-#endif
-
 // Make sure we don't get min/max macros
 #ifndef NOMINMAX
 #define NOMINMAX