GN: add sanitize arg

GN: add sanitize arg

Attempt to take over all *SAN builds.

MSAN has a lot of coordination required between gn/BUILD.gn and gn_flavor.py.
I'd like to follow up to move more of this into gn/BUILD.gn, to make it easier
to use locally.

The compile steps should be much faster now.  We no longer build CMake
and Clang for every run, instead using the clang_linux CIPD package.  This
removes the need for all the third_party/externals/llvm/... dependencies.

Similarly, since we're using the clang_linux package, we no longer depend
on Chrome's Clang, and thus no longer need to sync chromium on these bots.

Instead of packaging up MSAN libraries and llvm-symbolizer in the compile
output, I have the test / perf bots also depend on the clang_linux package.
These do not vary from build to build.

No more need for the xsan.blacklist -include hack: Clang, GN, and Ninja
all track changes to xsan.blacklist without our help.

This has the incidental effect of upgrading the compiler used by *SAN
bots from Clang 3.8 to Clang 3.9.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2289343002

Review-Url: https://codereview.chromium.org/2289343002