Add longClickView() to TestTouchUtils.

Often in tests we just need to click the middle of a view, so add a
method to do that.

R=dtrainor@chromium.org

Review URL: https://codereview.chromium.org/67813011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234646 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 234646