Add GM to test bitmap shaders with basic geometry and bitmap masks.

gyp/gmslides.gypi

 # include this gypi to include all the golden master slides.
 {
   'sources': [
     '../gm/aaclip.cpp',
     '../gm/aarectmodes.cpp',
     '../gm/alphagradients.cpp',
     '../gm/androidfallback.cpp',
     '../gm/arcofzorro.cpp',
     '../gm/arithmode.cpp',
     '../gm/beziereffects.cpp',
     '../gm/bicubicfilter.cpp',
     '../gm/bigblurs.cpp',
     '../gm/bigmatrix.cpp',
     '../gm/bigtext.cpp',
     '../gm/bitmapalphathreshold.cpp',
     '../gm/bitmapcopy.cpp',
     '../gm/bitmapmatrix.cpp',
     '../gm/bitmapfilters.cpp',
     '../gm/bitmappremul.cpp',
     '../gm/bitmaprect.cpp',
     '../gm/bitmaprecttest.cpp',
     '../gm/bitmapscroll.cpp',
+    '../gm/bitmapshader.cpp',
     '../gm/bleed.cpp',
     '../gm/blurs.cpp',
     '../gm/blurquickreject.cpp',
     '../gm/blurrect.cpp',
     '../gm/blurroundrect.cpp',
     '../gm/canvasstate.cpp',
     '../gm/circles.cpp',
     '../gm/circularclips.cpp',
     '../gm/clippedbitmapshaders.cpp',
     '../gm/coloremoji.cpp',
@@ skipping 110 matching lines @@
     '../gm/typeface.cpp',
     '../gm/verttext.cpp',
     '../gm/verttext2.cpp',
     '../gm/verylargebitmap.cpp',
     '../gm/xfermodeimagefilter.cpp',
     '../gm/xfermodes.cpp',
     '../gm/xfermodes2.cpp',
     '../gm/xfermodes3.cpp',
   ],
 }