Remove unused is_delta_update variable in drive::ChangeListProcessor.

BUG=none
R=hashimoto@chromium.org

Review URL: https://codereview.chromium.org/100713003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238906 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238906