Variations Trial: Reserve IDs for ExtensionInstallPrompt trial.

BUG=326733
NOTRY=true

Review URL: https://codereview.chromium.org/114893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241017 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241017