Purge image frame after checking for sufficient data.

It is possible to only have some of the image downloaded and attempt to
display an animation frame at the end. When this happens, we would purge
important frames.

BUG=652046

Review-Url: https://codereview.chromium.org/2382143004
Cr-Commit-Position: refs/heads/master@{#422605}

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp

@@ -335,12 +335,12 @@ void GIFImageDecoder::decode(size_t index) {
       return;
     }
 
-    if (m_purgeAggressively)
-      clearCacheExceptFrame(*i);
-
     // We need more data to continue decoding.
     if (m_frameBufferCache[*i].getStatus() != ImageFrame::FrameComplete)
       break;
+
+    if (m_purgeAggressively)
+      clearCacheExceptFrame(*i);
   }
 
   // It is also a fatal error if all data is received and we have decoded all