When performing offscreen rendering on windows, attempt to use a pbuffer context.

include/gpu/gl/SkGLContextHelper.h

Index: include/gpu/gl/SkGLContextHelper.h
diff --git a/include/gpu/gl/SkGLContextHelper.h b/include/gpu/gl/SkGLContextHelper.h
index 9da874184005a4f2abaa03c3968786779b1dfc22..a06c6e1f9b0a317654b3e484a2929f545f927539 100644
--- a/include/gpu/gl/SkGLContextHelper.h
+++ b/include/gpu/gl/SkGLContextHelper.h
@@ -78,9 +78,9 @@ private:
  * SK_GL(glCtx, GenTextures(1, &texID));
  */
 #define SK_GL(ctx, X) (ctx).gl()->fFunctions.f ## X;    \
-                      SkASSERT(GR_GL_NO_ERROR == (ctx).gl()->fFunctions.fGetError())
+                      SkASSERT(0 == (ctx).gl()->fFunctions.fGetError())
 #define SK_GL_RET(ctx, RET, X) (RET) = (ctx).gl()->fFunctions.f ## X;    \
-                  SkASSERT(GR_GL_NO_ERROR == (ctx).gl()->fFunctions.fGetError())
+                  SkASSERT(0 == (ctx).gl()->fFunctions.fGetError())
 #define SK_GL_NOERRCHECK(ctx, X) (ctx).gl()->fFunctions.f ## X
 #define SK_GL_RET_NOERRCHECK(ctx, RET, X) (RET) = (ctx).gl()->fFunctions.f ## X