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

Author: bsalomon@google.com

Reviewed By: robertphillips@google.com

Review URL: https://chromiumcodereview.appspot.com/13334011

git-svn-id: http://skia.googlecode.com/svn/trunk@8454 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/bench/benchmain.cpp b/bench/benchmain.cpp
index 4bc56c4..9f357c6 100644
--- a/bench/benchmain.cpp
+++ b/bench/benchmain.cpp
@@ -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) {