speed up benches: postpone hash computation in PictureRenderer.cpp

tools/PictureRenderer.cpp

Index: tools/PictureRenderer.cpp
diff --git a/tools/PictureRenderer.cpp b/tools/PictureRenderer.cpp
index 12f0afa879e04e5ee9ac2c8992e9331c9a8dca8f..b15b4a798338444a6de64782fa8b326d3dccb22d 100644
--- a/tools/PictureRenderer.cpp
+++ b/tools/PictureRenderer.cpp
@@ -298,9 +298,9 @@ static bool write(SkCanvas* canvas, const SkString& writePath, const SkString& m
     // TODO(epoger): what about including the config type within outputFilename?  That way,
     // we could combine results of different config types without conflicting filenames.
     SkString outputFilename;
-    const ImageDigest *imageDigestPtr = bitmapAndDigest.getImageDigestPtr();
     const char *outputSubdirPtr = NULL;
     if (useChecksumBasedFilenames) {
+        const ImageDigest *imageDigestPtr = bitmapAndDigest.getImageDigestPtr();
         outputSubdirPtr = escapedInputFilename.c_str();
         outputFilename.set(imageDigestPtr->getHashType());
         outputFilename.append("_");