Make the Sk GL context class an abstract base class

gyp/public_headers.gypi

 # Include this gypi to include all public header files that exist in the
 # include directory.
 #
 # The list is computed by running 'find include -name *.h' in the root dir of
 # the project.
 #
 {
   'variables': {
     'header_filenames': [
       'animator/SkAnimator.h',
@@ skipping 165 matching lines @@
       'gpu/SkGr.h',
       'gpu/SkGrPixelRef.h',
       'gpu/SkGrTexturePixelRef.h',
       'gpu/gl/GrGLConfig.h',
       'gpu/gl/GrGLConfig_chrome.h',
       'gpu/gl/GrGLExtensions.h',
       'gpu/gl/GrGLFunctions.h',
       'gpu/gl/GrGLInterface.h',
       'gpu/gl/SkANGLEGLContext.h',
       'gpu/gl/SkDebugGLContext.h',
-      'gpu/gl/SkGLContextHelper.h',
       'gpu/gl/SkMesaGLContext.h',
-      'gpu/gl/SkNativeGLContext.h',
+      'gpu/gl/SkGLContext.h',
       'gpu/gl/SkNullGLContext.h',
       'images/SkForceLinking.h',
       'images/SkMovie.h',
       'images/SkPageFlipper.h',
       'pathops/SkPathOps.h',
       'pdf/SkPDFDevice.h',
       'pdf/SkPDFDocument.h',
       'pipe/SkGPipe.h',
       'ports/SkFontConfigInterface.h',
       'ports/SkFontMgr.h',
@@ skipping 68 matching lines @@
       'views/unix/keysym2ucs.h',
       'xml/SkBML_WXMLParser.h',
       'xml/SkBML_XMLParser.h',
       'xml/SkDOM.h',
       'xml/SkJS.h',
       'xml/SkXMLParser.h',
       'xml/SkXMLWriter.h',
     ],
   },
 }