We would accidentally propagate include directories to the test
main target. This would show up as compile errors of the form:

In file included from ../../third_party/angle/testing/gtest/include/gtest/gtest.h:10:
../../third_party/angle/third_party/googletest/src/googletest/include/gtest/gtest.h:303:22: error: no member named 'ImplicitlyConvertible' in namespace 'testing::internal'
          !internal::ImplicitlyConvertible<T, AssertionResult>::value>::type*
           ~~~~~~~~~~^

This fixes the GN properties such that we no longer get the wrong
/testing/ and /third_party/googletest/ directories included. This
regressed when glslang was rolled.

Bug: angleproject:3088
Change-Id: I710f767cfe476454c10dacf2103b9203f1b5ee19
Reviewed-on: https://chromium-review.googlesource.com/c/1459596Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>