[Zucchini] Update DEPS entry for test data download.

- Move bucket to gs://chromium-binary-patching/zucchini-testdata, which
  is more specific, and has a manifest file:
  gsutil cat gs://chromium-binary-patching/zucchini_testdata/manifest.json
  - Old file copies are kept in root as legacy.
- Move destination path to Zucchini root, and make it recursively look
  for .sha1 files. The reaosn is that we'll add fuzzing data in a
  different directory, and want to use the same hook name.

Details: 
Change-Id: Ie96cb99de9e0727d3cb6dfb0afa25c08cf4f31c3
Reviewed-on: https://chromium-review.googlesource.com/1038047Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555709}

DEPS

@@ -1876,8 +1876,9 @@ hooks = [
                 '--no_resume',
                 '--no_auth',
                 '--num_threads=4',
-                '--bucket', 'chromium-binary-patching',
-                '-d', 'src/components/zucchini/testdata',
+                '--bucket', 'chromium-binary-patching/zucchini_testdata',
+                '--recursive',
+                '-d', 'src/components/zucchini',
     ],
   },