Use ninja with merge_libs.py on Windows and clean up.
Use ninja with merge_libs.py on Windows and clean up.

BUG=b/8110634

Review URL: https://webrtc-codereview.appspot.com/1094006

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: dd5482cd2f476baeadc86e61dfd93791e6186017

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc