Add Xfermode bench. Also clear before rendering in bench (rather than after).

bench/benchmain.cpp

Index: bench/benchmain.cpp
===================================================================
--- bench/benchmain.cpp	(revision 8451)
+++ bench/benchmain.cpp	(working copy)
@@ -822,6 +822,7 @@
                         SkASSERT(0);
                 }
                 device->unref();
+                canvas->clear(SK_ColorWHITE);
             }
             SkAutoUnref canvasUnref(canvas);
 
@@ -928,7 +929,6 @@
             if (outDir.size() > 0 && kNonRendering_Backend != backend) {
                 saveFile(bench->getName(), configName, outDir.c_str(),
                          device->accessBitmap(false));
-                canvas->clear(SK_ColorWHITE);
             }
         }
         if (loggedBenchStart) {