Add missing fallback blending to SkA8_Shader_Blitter::blitMask()

Similarly to SkA8_Shader_Blitter::blitAntiH() and SkA8_Shader_Blitter::blitH(), blitMask() should handle a missing SkXfermode gracefully.

R=reed@google.com

Author: fmalita@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22867008

git-svn-id: http://skia.googlecode.com/svn/trunk/src@10743 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkBlitter_A8.cpp b/core/SkBlitter_A8.cpp
index 0255464..7fca29e 100644
--- a/core/SkBlitter_A8.cpp
+++ b/core/SkBlitter_A8.cpp
@@ -336,6 +336,10 @@
         fShader->shadeSpan(x, y, span, width);
         if (fXfermode) {
             fXfermode->xferA8(device, span, width, alpha);
+        } else {
+            for (int i = width - 1; i >= 0; --i) {
+                device[i] = aa_blend8(span[i], device[i], alpha[i]);
+            }
         }
 
         y += 1;