Add a sanitizer.gni import to the GN build.

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

R=brettw@chromium.org, harryjin@google.com
BUG=none

Review URL: https://codereview.chromium.org/1314833005 .

BUILD.gn

@@ -7,6 +7,7 @@
 # be found in the AUTHORS file in the root of the source tree.
 
 import("//build/config/arm.gni")
+import("//build/config/sanitizers/sanitizers.gni")
 
 config("libyuv_config") {
   include_dirs = [