Use glGetStringi to get extensions when available.

gyp/gpu.gypi

 # Include this gypi to include all 'gr' and 'skgr' files
 # The parent gyp/gypi file must define
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi
 #
 {
   'variables': {
     'gr_sources': [
@@ skipping 20 matching lines @@
       '<(skia_include_path)/gpu/GrResource.h',
       '<(skia_include_path)/gpu/GrSurface.h',
       '<(skia_include_path)/gpu/GrTBackendEffectFactory.h',
       '<(skia_include_path)/gpu/GrTextContext.h',
       '<(skia_include_path)/gpu/GrTexture.h',
       '<(skia_include_path)/gpu/GrTextureAccess.h',
       '<(skia_include_path)/gpu/GrTypes.h',
       '<(skia_include_path)/gpu/GrUserConfig.h',
 
       '<(skia_include_path)/gpu/gl/GrGLConfig.h',
+      '<(skia_include_path)/gpu/gl/GrGLExtensions.h',
       '<(skia_include_path)/gpu/gl/GrGLFunctions.h',
       '<(skia_include_path)/gpu/gl/GrGLInterface.h',
 
       '<(skia_src_path)/gpu/GrAAHairLinePathRenderer.cpp',
       '<(skia_src_path)/gpu/GrAAHairLinePathRenderer.h',
       '<(skia_src_path)/gpu/GrAAConvexPathRenderer.cpp',
       '<(skia_src_path)/gpu/GrAAConvexPathRenderer.h',
       '<(skia_src_path)/gpu/GrAARectRenderer.cpp',
       '<(skia_src_path)/gpu/GrAddPathRenderers_default.cpp',
       '<(skia_src_path)/gpu/GrAllocator.h',
@@ skipping 87 matching lines @@
       '<(skia_src_path)/gpu/gl/GrGLBufferImpl.h',
       '<(skia_src_path)/gpu/gl/GrGLCaps.cpp',
       '<(skia_src_path)/gpu/gl/GrGLCaps.h',
       '<(skia_src_path)/gpu/gl/GrGLContextInfo.cpp',
       '<(skia_src_path)/gpu/gl/GrGLContextInfo.h',
       '<(skia_src_path)/gpu/gl/GrGLCreateNativeInterface_none.cpp',
       '<(skia_src_path)/gpu/gl/GrGLDefaultInterface_none.cpp',
       '<(skia_src_path)/gpu/gl/GrGLDefines.h',
       '<(skia_src_path)/gpu/gl/GrGLEffect.cpp',
       '<(skia_src_path)/gpu/gl/GrGLEffect.h',
+      '<(skia_src_path)/gpu/gl/GrGLExtensions.cpp',
       '<(skia_src_path)/gpu/gl/GrGLEffectMatrix.cpp',
       '<(skia_src_path)/gpu/gl/GrGLEffectMatrix.h',
       '<(skia_src_path)/gpu/gl/GrGLIndexBuffer.cpp',
       '<(skia_src_path)/gpu/gl/GrGLIndexBuffer.h',
       '<(skia_src_path)/gpu/gl/GrGLInterface.cpp',
       '<(skia_src_path)/gpu/gl/GrGLIRect.h',
       '<(skia_src_path)/gpu/gl/GrGLNoOpInterface.cpp',
       '<(skia_src_path)/gpu/gl/GrGLNoOpInterface.h',
       '<(skia_src_path)/gpu/gl/GrGLPath.cpp',
       '<(skia_src_path)/gpu/gl/GrGLPath.h',
@@ skipping 104 matching lines @@
       '<(skia_src_path)/gpu/gl/SkNullGLContext.cpp',
     ],
   },
 }
 
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: