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}