x11: Cleanup how valuators are set for X events in tests.

BUG=none
R=derat@chromium.org

Review URL: https://codereview.chromium.org/95403006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238264 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238264