Add a sanitizer.gni import to the GN build.

The is_*san build flags are moving to this .gni file. This patch will allow
libvpx to continue building when this move happens.

R=tomfinegan@chromium.org

Review URL: https://codereview.chromium.org/1320303007 .
diff --git a/BUILD.gn b/BUILD.gn
index ab50ae7..2385163 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/config/arm.gni")
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
+import("//build/config/sanitizers/sanitizers.gni")
 import("//third_party/libvpx/libvpx_srcs.gni")
 import("//third_party/yasm/yasm_assemble.gni")