Specify binary directory only for perl used in webkit layout test on win

This is to reduce the number of files sent to swarming under third_party/perl directory.
e.g.
https://isolateserver.appspot.com/browse?namespace=default-gzip&hash=91c35f024b8949020a3a25f3421f27689f35dff5
from
https://chromium-swarm.appspot.com/task?id=40da578fadf59310&refresh=10&show_raw=1

Bug: 899991
Change-Id: I8608283399e817c8d3d8558782d67587a735e96a
Reviewed-on: https://chromium-review.googlesource.com/c/1331097Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607227}

BUILD.gn

@@ -1017,7 +1017,7 @@ if (!is_ios) {
     if (is_win) {
       data += [
         "//third_party/apache-win32/",
-        "//third_party/perl/",
+        "//third_party/perl/perl/",
       ]
     }