GS:// ; Convert the atomic creation code to use generation.

BUG=chromium-os:37718
TEST=manual validation of the behaviour

Change-Id: I8c7fc099fc91c760eb2083dadc607ce4e095c28c
Previous-Reviewed-on: https://gerrit.chromium.org/gerrit/40928
(cherry picked from commit 71bd9b11f689000c64f6c411a5f8765c43962bdb)
Previous-Reviewed-on: https://gerrit.chromium.org/gerrit/41105
Commit-Queue: Jon Salz <jsalz@chromium.org>
(cherry picked from commit a11e59dce372e5c0d13759ffa8602e0c8ea01478)
Reviewed-on: https://gerrit.chromium.org/gerrit/41183
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
diff --git a/buildbot/manifest_version.py b/buildbot/manifest_version.py
index cbd26e3..8c7cc1b 100644
--- a/buildbot/manifest_version.py
+++ b/buildbot/manifest_version.py
@@ -647,7 +647,7 @@
     if fail_if_exists:
       # This HTTP header tells Google Storage toreturn the PreconditionFailed
       # error message if the file already exists.
-      cmd += ['-h', 'x-goog-if-sequence-number-match: 0']
+      cmd += ['-h', 'x-goog-if-generation-match: 0']
     url = self._GetStatusUrl(self.build_name, version)
     cmd += ['cp', '-', url]