Factor out checkerboard function in gm and sampleapp into tools.

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',
   ],
+  'dependencies': [
+     'tools.gyp:checkerboard',
+  ],
   'conditions': [
     # If we're building SampleApp on the bots, no need to link in the GM slides.
     # We're not going to run it; we're only making sure it still builds.
     # It'd be nice to do this in SampleApp.gypi, but I can't find a way to make it work.
     [ 'not ("<(_target_name)" == "SampleApp" and skia_is_bot)', {
       'sources': [
         '../gm/aaclip.cpp',
         '../gm/aarectmodes.cpp',
         '../gm/alphagradients.cpp',
         '../gm/arcofzorro.cpp',
@@ skipping 227 matching lines @@
         '../gm/shadertext2.cpp',
         '../gm/shadertext3.cpp',
 
         # TODO(reed): Allocates more memory than Android devices are capable of
         # fulfilling. See http://skbug.com/1978
         '../gm/verylargebitmap.cpp',
       ],
     }],
   ],
 }