Gracefully handle FileReader() read operations when in a detached state.

Source/core/fileapi/FileReader.cpp

Index: Source/core/fileapi/FileReader.cpp
diff --git a/Source/core/fileapi/FileReader.cpp b/Source/core/fileapi/FileReader.cpp
index 7e7232e71e36e49b2ab577248c24ad8470085f5e..91c1c3bc9afda0d45f6eea5300f06ae16a203209 100644
--- a/Source/core/fileapi/FileReader.cpp
+++ b/Source/core/fileapi/FileReader.cpp
@@ -80,6 +80,9 @@ class FileReader::ThrottlingController FINAL : public NoBaseWillBeGarbageCollect
 public:
     static ThrottlingController* from(ExecutionContext* context)
     {
+        if (!context)
+            return 0;
+
         if (context->isDocument()) {
             Document* document = toDocument(context);
             if (!document->frame())
@@ -285,6 +288,11 @@ void FileReader::readInternal(Blob* blob, FileReaderLoader::ReadType type, Excep
         return;
     }
 
+    if (!throttlingController()) {
+        exceptionState.throwDOMException(NotSupportedError, "Reading from a Document-detached FileReader is not supported.");

[kinuko, 2014/07/17 15:22:24]

I don't have strong opinion, but AbortError might sound more common in these
cases?

+        return;
+    }
+
     // "Snapshot" the Blob data rather than the Blob itself as ongoing
     // read operations should not be affected if close() is called on
     // the Blob being read.