[swarming] Isolate mac asan.

BUG=chromium:535160
LOG=n
NOTRY=true
TBR=tandrii@chromium.org, kjellander@chromium.org

Review URL: https://codereview.chromium.org/1573913003

Cr-Commit-Position: refs/heads/master@{#33210}

src/base.isolate

@@ -38,6 +38,13 @@
         ],
       },
     }],
+    ['OS=="mac" and asan==1', {
+      'variables': {
+        'files': [
+          '<(PRODUCT_DIR)/libclang_rt.asan_osx_dynamic.dylib',
+        ],
+      },
+    }],
     ['tsan==1', {
       'variables': {
         'files': [
@@ -65,4 +72,4 @@
       'variables': {},
     }],
   ],
-}
\ No newline at end of file
+}