Revert[8] "replace SkXfermode obj with SkBlendMode enum in paints"

This reverts commit c245574ba3d0e2ade6c94b2812de3baa383bf4c4.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2396953002

TBR=

Review-Url: https://codereview.chromium.org/2396953002
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index a37a6ff..52e063d 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -352,7 +352,7 @@
             bool doGamma = (fActualColorBits == 30) && SkImageInfoIsGammaCorrect(win->info());
 
             SkPaint gammaPaint;
-            gammaPaint.setXfermodeMode(SkXfermode::kSrc_Mode);
+            gammaPaint.setBlendMode(SkBlendMode::kSrc);
             if (doGamma) {
                 gammaPaint.setColorFilter(SkGammaColorFilter::Make(1.0f / 2.2f));
             }