Fix fuzzer build on Mac for ICU

Make icu depend on //buildtools/third_party/libc++abi
on mac for sanitizer builds.

BUG=636127
TEST=Libfuzzer Upload Mac ASan builds
R=mmoroz@chromium.org

Review URL: https://codereview.chromium.org/2247953005 .
diff --git a/BUILD.gn b/BUILD.gn
index 3032b50..1dd7c61 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,6 +8,10 @@
   import("//build/config/android/rules.gni")
 }
 
+if (is_mac) {
+  import("//build/config/sanitizers/sanitizers.gni")
+}
+
 # Meta target that includes both icuuc and icui18n. Most targets want both.
 # You can depend on the individually if you need to.
 group("icu") {
@@ -954,6 +958,10 @@
     ":icudata",
   ]
 
+  if (is_mac && using_sanitizer) {
+    deps += [ "//buildtools/third_party/libc++abi" ]
+  }
+
   configs -= [
     "//build/config/compiler:no_rtti",  # ICU uses RTTI.
     "//build/config/compiler:chromium_code",