Work around exasperating Google3 Android build error.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1649493005

Review URL: https://codereview.chromium.org/1649493005

include/core/SkXfermode.h

@@ -246,6 +246,9 @@ private:
         kModeCount = kLastMode + 1
     };
 
+    // TODO(benjaminwagner): This is crazy, but it fixes the Google3 Android build. :-(
+    virtual void doNothing() const;
+
     typedef SkFlattenable INHERITED;
 };
 

src/core/SkXfermode.cpp

@@ -1071,3 +1071,5 @@ bool SkXfermode::IsOpaque(const SkXfermode* xfer, SrcColorOpacity opacityType) {
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkXfermode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkProcCoeffXfermode)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END
+
+void SkXfermode::doNothing() const {}