speed up benches: postpone hash computation in PictureRenderer.cpp

fixes phantom performance regression after https://codereview.chromium.org/273783004

R=borenet@google.com, bensong@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/299763002

git-svn-id: http://skia.googlecode.com/svn/trunk@14819 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/PictureRenderer.cpp b/tools/PictureRenderer.cpp
index 12f0afa..b15b4a7 100644
--- a/tools/PictureRenderer.cpp
+++ b/tools/PictureRenderer.cpp
@@ -298,9 +298,9 @@
     // 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("_");