Add a GN import for sanitizers.gni.

The is_*san flags are moving from the master build config file into this
.gni file. This patch will allow V8 to continue compiling when that change
is landed.

Review URL: https://codereview.chromium.org/1330713003

Cr-Commit-Position: refs/heads/master@{#30623}

BUILD.gn

@@ -5,6 +5,7 @@
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
+import("//build/config/sanitizers/sanitizers.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.