Adding unit test for verifying copy on write in SkSurface + bug fix in SkCanvas::clear

gyp/tests.gyp

 # GYP file to build unit tests.
 {
   'includes': [
     'apptype_console.gypi',
   ],
   'targets': [
     {
       'target_name': 'tests',
       'type': 'executable',
       'include_dirs' : [
@@ skipping 80 matching lines @@
         '../tests/ScalarTest.cpp',
         '../tests/ShaderImageFilterTest.cpp',
         '../tests/ShaderOpacityTest.cpp',
         '../tests/Sk64Test.cpp',
         '../tests/skia_test.cpp',
         '../tests/SortTest.cpp',
         '../tests/SrcOverTest.cpp',
         '../tests/StreamTest.cpp',
         '../tests/StringTest.cpp',
         '../tests/StrokeTest.cpp',
+        '../tests/SurfaceTest.cpp',
         '../tests/Test.cpp',
         '../tests/Test.h',
         '../tests/TestSize.cpp',
         '../tests/TileGridTest.cpp',
         '../tests/TLSTest.cpp',
         '../tests/TSetTest.cpp',
         '../tests/ToUnicode.cpp',
         '../tests/UnicodeTest.cpp',
         '../tests/UtilsTest.cpp',
         '../tests/WArrayTest.cpp',
@@ skipping 31 matching lines @@
       ],
     },
   ],
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: