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

As a temporary solution, this CL introduces gtest as a secondary build
source. For now, gtest can be included in testonly targets as a direct
GN target dependency, by placing the gtest source under "testing/gtest"
at the root of the tree.

Moving forward, we can use solutions like gclient to pull in external
dependencies such as gtest, or we can depend on it being available on
the system.

BUG=21339022,21340407

Change-Id: I80ab3a81a53d7c29d54857a8d0cd9677fed77549

BUG=20491586