Small DM cleanup

We set a value to bounds, and then immediately replace it. Skip the
first one.

Review URL: https://codereview.chromium.org/1378923002
diff --git a/dm/DMSrcSink.cpp b/dm/DMSrcSink.cpp
index d98a9ca..2ea115b 100644
--- a/dm/DMSrcSink.cpp
+++ b/dm/DMSrcSink.cpp
@@ -441,8 +441,7 @@
                     }
                     //create and set size of subsetBm
                     SkBitmap subsetBm;
-                    SkIRect bounds = SkIRect::MakeWH(subsetWidth, subsetHeight);
-                    bounds.setXYWH(0, 0, currentSubsetWidth, currentSubsetHeight);
+                    SkIRect bounds = SkIRect::MakeWH(currentSubsetWidth, currentSubsetHeight);
                     SkAssertResult(largestSubsetBm.extractSubset(&subsetBm, bounds));
                     SkAutoLockPixels autlockSubsetBm(subsetBm, true);
                     const SkCodec::Result subsetResult =