Disable sanitizers with a blacklist.

We think this might be more flexible.  It allows, e.g, function-level blacklisting,
and here an easy one-stop-shop blacklist for all of third_party/externals.

BUG=skia:

CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Debug-ASAN-Trybot
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/1509733003
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 057e936..2980f5b 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -202,6 +202,8 @@
     'skia_no_fontconfig%': '<(skia_no_fontconfig)',
     'skia_embedded_fonts%': '<(skia_embedded_fonts)',
     'skia_sanitizer%': '<(skia_sanitizer)',
+    'skia_sanitizer_blacklist%':
+      '<!(python -c "import sys; import os; print os.path.abspath(sys.argv[1])" ../tools/xsan.blacklist)',
     'skia_mesa%': '<(skia_mesa)',
     'skia_gpu_extra_dependency_path%': '<(skia_gpu_extra_dependency_path)',
     'skia_gpu_extra_tests_path%': '<(skia_gpu_extra_tests_path)',