Switching Breakpad from configure to gn build

[Zequan Wu]

Hi,

Currently there are two ways to build Breakpad, `./configure && make` and gn build from  chromium project. And some unittests are only available using configure and make. Switching breakpad from configure to gn build would make building and testing much easier. 

Zequan Wu

[Benjamin Sergeant]

make is a very common build system while gn isn't, so for ease of use with outside projects it wouldn't be ideal.