Use system unzip on macOS

Modern macOS has no problem unzipping big zip files. To test:

$ gsutil cp gs://chromium-git-cache/chromium.googlesource.com-chromium-src/516491.zip test.zip
$ unzip -l test.zip

Change-Id: I84b3cff21cb9b7033c04b427e23f27a75ab1d8ae
Reviewed-on: https://chromium-review.googlesource.com/1152294
Commit-Queue: Jeremy Apthorp <jeremya@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

git_cache.py

@@ -357,15 +357,12 @@ class Mirror(object):
     """
     if not self.bootstrap_bucket:
       return False
-    python_fallback = False
-    if (sys.platform.startswith('win') and
-        not gclient_utils.FindExecutable('7z')):
-      python_fallback = True
-    elif sys.platform.startswith('darwin'):
-      # The OSX version of unzip doesn't support zip64.
-      python_fallback = True
-    elif not gclient_utils.FindExecutable('unzip'):
-      python_fallback = True
+    python_fallback = (
+        (sys.platform.startswith('win') and
+          not gclient_utils.FindExecutable('7z')) or
+        (not gclient_utils.FindExecutable('unzip')) or
+        ('ZIP64_SUPPORT' not in subprocess.check_output(["unzip", "-v"]))
+    )
 
     gs_folder = 'gs://%s/%s' % (self.bootstrap_bucket, self.basedir)
     gsutil = Gsutil(self.gsutil_exe, boto_path=None)