Update gsutil pin for download_from_google_storage.py from 4.15 to 4.26

This picks up about 2 years worth of changes.

Bug:723733
TEST=Ran gclient sync on src.git checkout, worked.
Change-Id: I7021ac62be560bb3ba7b523f0758d56bceeef12a
Reviewed-on: https://chromium-review.googlesource.com/527262
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>

download_from_google_storage.py

@@ -60,7 +60,7 @@ class Gsutil(object):
   RETRY_BASE_DELAY = 5.0
   RETRY_DELAY_MULTIPLE = 1.3
 
-  def __init__(self, path, boto_path=None, timeout=None, version='4.15'):
+  def __init__(self, path, boto_path=None, timeout=None, version='4.26'):
     if not os.path.exists(path):
       raise FileNotFoundError('GSUtil not found in %s' % path)
     self.path = path