1.
BUILD.gn: Don't use the extra Chromium clang warnings
Also removes the unused .gn secondary_sources.
2.
Move fuzzers in test/ instead of testing/
This frees up testing/ to be the git subtree of Chromium's src/testing/
that contains test.gni, gtest, gmock and libfuzzer
3.
DEPS: get the whole testing/ subtree of Chromium
4.
BUILD.gn: Simplify the standalone gtest targets
These targets definitions are inspired from ANGLE's and add a variable
that is the path of the googletest directory so that it can be made
overridable in future commits.
6.
BUILD.gn: Add overridable variables for deps dirs
This avoids hardcoded paths to dependencies that make it hard to
integrate SPIRV-Tools in other GN projects.
