Using self.GetDescription() uses the description from the web.
Given that the user is purposefully disassociating their current
branch from the review on the web, they may have already changed
the commit message. We shouldn't set it back.

Bug: 742730
Change-Id: I0545cb6288c332fd475d1de7fb302f71ee41a415
Reviewed-on: https://chromium-review.googlesource.com/578229Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>