Fix clang 'overloaded-virtual' warnings.

Fix clang 'overloaded-virtual' warnings.

This patch fixes the warning like the following:

../../src/animator/SkPaintParts.h:26:18: warning: 'SkPaintPart::add' hides overloaded virtual function [-Woverloaded-virtual]
virtual bool add() = 0;
             ^
../../src/animator/SkDisplayable.h:35:18: note: hidden overloaded virtual function 'SkDisplayable::add' declared here
virtual bool add(SkAnimateMaker& , SkDisplayable* child);
             ^

R=robertphillips@google.com,

Review URL: https://codereview.appspot.com/6843134

git-svn-id: http://skia.googlecode.com/svn/trunk@6642 2bbb7eff-a529-9590-31e7-b0007b416f81