Add support for blending of LCD for all blend modes.
BUG=skia:

Review URL: https://codereview.chromium.org/1313623002
diff --git a/src/gpu/GrTextContext.cpp b/src/gpu/GrTextContext.cpp
index 92a851b..be62619 100644
--- a/src/gpu/GrTextContext.cpp
+++ b/src/gpu/GrTextContext.cpp
@@ -80,7 +80,7 @@
 
 bool GrTextContext::ShouldDisableLCD(const SkPaint& paint) {
     if (paint.getShader() ||
-        !SkXfermode::IsMode(paint.getXfermode(), SkXfermode::kSrcOver_Mode) ||
+        !SkXfermode::AsMode(paint.getXfermode(), nullptr) ||
         paint.getMaskFilter() ||
         paint.getRasterizer() ||
         paint.getColorFilter() ||