Fix race conditions in ImageDecoder.

chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc

Index: chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc
diff --git a/chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc b/chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc
index ac590cd73d7fd7946ff41515855bdf1c882e7596..2e83a76f3557547038f6dbe30cc5d81a5627fcf4 100644
--- a/chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc
+++ b/chrome/browser/media_galleries/fileapi/supported_image_type_validator.cc
@@ -52,9 +52,7 @@ class ImageDecoderDelegateAdapter : public ImageDecoder::ImageRequest {
   ImageDecoderDelegateAdapter(
       scoped_ptr<std::string> data,
       const storage::CopyOrMoveFileValidator::ResultCallback& callback)
-      : ImageRequest(content::BrowserThread::GetMessageLoopProxyForThread(
-            BrowserThread::IO)),

[Theresa, 2015/04/20 22:35:26]

I'm not familiar enough with this class to know whether or not it's constructed
on the IO thread... or whether or not OnImageDecoded/OnDecodeImageFailed
actually need to be called on the IO thread since they just invoke
CopyOrMoveFileValidator::ResultCallback.Run()?

[Lei Zhang, 2015/04/21 00:44:00]

The only place that calls the ImageDecoderDelegateAdapter ctor is in
SupportedImageTypeValidator::OnFileOpen(), which clearly runs on the IO thread.

-        data_(data.Pass()),
+      : data_(data.Pass()),
         callback_(callback) {
     DCHECK(data_);
   }