gn-build: Add tinyxml2 as a secondary source

Added tinyxml2 as a secondary source. Moved //build/secondary/testing to
//build/secondary/third_party in the process. This means that all third_party
sources (gtest, tinyxml2) should be placed in a top-level third_party directory.

BUG=21339022

Change-Id: If324ef1eb0d5061e005f62a80d23fef25e7e78a4
4 files changed