Rewrite Microsoft Windows build to use gn and match other builds.

[DanielTBrown]

This update has the following changes:

build.sh:

• added '-b' to fetch a branch by name
• removed Microsoft Visual C++ version option, since the WebRTC build supports only Microsoft Visual C++ 2015

util.sh

• added a Microsoft Windows build function to use 'gn', matching the other builds
• no changes to builds for hosts other than Microsoft Windows

Unfortunately, I can only partially test the Microsoft Windows build, because my Microsoft Windows development system encounters an error in landmines.py. This is unrelated to the script, I see the same error when building manually. When I have a chance, I will test using a clean Windows install.

[vsimon]

Looks great to me!