Add GM that exercises SkCanvas::discard()

gyp/gmslides.gypi

 # include this gypi to include all the golden master slides.
 {
   'include_dirs': [
     '../gm',
     # include dirs needed by particular GMs
     '../src/utils/debugger',
     '../src/images',
     '../src/lazy',
   ],
   'sources': [
@@ skipping 38 matching lines @@
     '../gm/complexclip2.cpp',
     '../gm/composeshader.cpp',
     #'../gm/conicpaths.cpp',
     '../gm/convexpaths.cpp',
     '../gm/convexpolyclip.cpp',
     '../gm/convexpolyeffect.cpp',
     '../gm/copyTo4444.cpp',
     '../gm/cubicpaths.cpp',
     '../gm/cmykjpeg.cpp',
     '../gm/degeneratesegments.cpp',
+    '../gm/discard.cpp',
     '../gm/dashcubics.cpp',
     '../gm/dashing.cpp',
     '../gm/deviceproperties.cpp',
     '../gm/distantclip.cpp',
     '../gm/displacement.cpp',
     '../gm/downsamplebitmap.cpp',
     '../gm/drawbitmaprect.cpp',
     '../gm/drawlooper.cpp',
     '../gm/dropshadowimagefilter.cpp',
     '../gm/drrect.cpp',
@@ skipping 152 matching lines @@
         # fulfilling. See http://skbug.com/1978
         '../gm/verylargebitmap.cpp',
       ],
 
       'sources': [
         '../gm/androidfallback.cpp',
       ],
     }],
   ],
 }