Remove unused variable OcclusionTracker::prevent_occlusion.

It has been replaced by RenderSurface::nearest_occlusion_immune_ancestor.

Review URL: https://codereview.chromium.org/74923004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235843 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235843