Destroy SkScanlineDecoder in the SkCodec subclass destructors

include/codec/SkScanlineDecoder.h

Index: include/codec/SkScanlineDecoder.h
diff --git a/include/codec/SkScanlineDecoder.h b/include/codec/SkScanlineDecoder.h
index d7f73dda56f92ef4bccc7bc431282b43d05a05a0..009fbec3177af791dfe38e95de48a8af56591d87 100644
--- a/include/codec/SkScanlineDecoder.h
+++ b/include/codec/SkScanlineDecoder.h
@@ -17,7 +17,9 @@ class SkScanlineDecoder : public SkNoncopyable {
 public:
     // Note for implementations: An SkScanlineDecoder will be deleted by (and
     // therefore *before*) its associated SkCodec, in case the order matters.
-    virtual ~SkScanlineDecoder() {}
+    virtual ~SkScanlineDecoder() {
+        this->onFinish();
+    }
 
     /**
      *  Write the next countLines scanlines into dst.
@@ -34,7 +36,7 @@ public:
             return SkImageGenerator::kInvalidParameters;
         }
         const SkImageGenerator::Result result = this->onGetScanlines(dst, countLines, rowBytes);
-        this->checkForFinish(countLines);
+        fCurrScanline += countLines;
         return result;
     }
 
@@ -54,7 +56,7 @@ public:
             return SkImageGenerator::kInvalidParameters;
         }
         const SkImageGenerator::Result result = this->onSkipScanlines(countLines);
-        this->checkForFinish(countLines);
+        fCurrScanline += countLines;
         return result;
     }
 
@@ -98,19 +100,11 @@ private:
                                                     size_t rowBytes) = 0;
 
     /**
-     *  Called after any set of scanlines read/skipped. Updates fCurrScanline,
-     *  and, if we are at the end, calls onFinish().
-     */
-    void checkForFinish(int countLines) {
-        fCurrScanline += countLines;
-        if (fCurrScanline >= fDstInfo.height()) {
-            this->onFinish();
-        }
-    }
-
-    /**
-     *  This function will be called after reading/skipping all scanlines to do
+     *  This function will be called after reading/skipping scanlines to do
      *  any necessary cleanups.
+     *  It is possible that not all scanlines will have been read/skipped when this
+     *  function is called.  In fact, in the case of subset decodes, it is likely
+     *  that there will be scanlines at the bottom of the image that have been ignored.
      */
     virtual void onFinish() {} // Default does nothing.
 };