Add data dependency directories for blink_tests in BUILD.gn.

Suggested in http://crrev.com/2190163003. This should make it so that the analyze step identifies blink_tests as a dependency for changes in layout tests and WebKit/Tools/Scripts.

BUG=631014

Review-Url: https://codereview.chromium.org/2195223002
Cr-Commit-Position: refs/heads/master@{#408996}
diff --git a/BUILD.gn b/BUILD.gn
index f44054cc..b9b9233 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -917,6 +917,11 @@
     if (is_linux) {
       deps += [ "//breakpad:dump_syms($host_toolchain)" ]
     }
+
+    data = [
+      "//third_party/WebKit/LayoutTests/",
+      "//third_party/WebKit/Tools/Scripts/",
+    ]
   }
 }