Fix MSVC warnings about value truncations, webrtc/base/ edition.

BUG=chromium:81439
TEST=none
R=henrike@webrtc.org, marpan@google.com

Review URL: https://webrtc-codereview.appspot.com/20249004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7143 4adac7df-926f-26a2-2b94-8c16560cd09d