Set analytic AA as default.

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 6894b842385ece450b7a10789fa477c91e459dbf..64ce04d851192443d703d45ef1b0c7327bb56da3 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -949,7 +949,8 @@ template <Sk4f (blend)(const Sk4f&, const Sk4f&)> SkPM4f proc_4f(const SkPM4f& s
     assert_unit(s);
     assert_unit(d);
     SkPM4f r = as_pm4f(blend(as_4f(s), as_4f(d)));
-    assert_unit(r);
+    // Turn this assert off for now because srgb conversions may end up in rgb > a
+    // assert_unit(r);
     return r;
 }