Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

gyp/core.gypi

Index: gyp/core.gypi
diff --git a/gyp/core.gypi b/gyp/core.gypi
index e98400f3bd17eb9df0871a49ab44695b20ce9e71..40d8c5c7abf35cc2c32f77f7e1580cdce388a23f 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -211,6 +211,7 @@
         '<(skia_src_path)/image/SkImage_Picture.cpp',
         '<(skia_src_path)/image/SkImage_Raster.cpp',
         '<(skia_src_path)/image/SkSurface.cpp',
+        '<(skia_src_path)/image/SkSurface_Base.h',
 #        '<(skia_src_path)/image/SkSurface_Gpu.cpp',
         '<(skia_src_path)/image/SkSurface_Picture.cpp',
         '<(skia_src_path)/image/SkSurface_Raster.cpp',
@@ -250,6 +251,7 @@
         '<(skia_include_path)/core/SkFontHost.h',
         '<(skia_include_path)/core/SkGeometry.h',
         '<(skia_include_path)/core/SkGraphics.h',
+        '<(skia_include_path)/core/SkImage.h',
         '<(skia_include_path)/core/SkImageDecoder.h',
         '<(skia_include_path)/core/SkImageEncoder.h',
         '<(skia_include_path)/core/SkImageFilter.h',
@@ -285,6 +287,7 @@
         '<(skia_include_path)/core/SkString.h',
         '<(skia_include_path)/core/SkStringUtils.h',
         '<(skia_include_path)/core/SkStrokeRec.h',
+        '<(skia_include_path)/core/SkSurface.h',
         '<(skia_include_path)/core/SkTArray.h',
         '<(skia_include_path)/core/SkTDArray.h',
         '<(skia_include_path)/core/SkTDStack.h',