Introduce FetchFormDataConsumerHandle.

Source/modules/fetch/FetchDataConsumerHandle.h

Index: Source/modules/fetch/FetchDataConsumerHandle.h
diff --git a/Source/modules/fetch/FetchDataConsumerHandle.h b/Source/modules/fetch/FetchDataConsumerHandle.h
index 6f811ab5398541ab309f4cf903f72641f99b1fe8..5703a4cae28e1b0b36d1c92f9c1954b81363fce1 100644
--- a/Source/modules/fetch/FetchDataConsumerHandle.h
+++ b/Source/modules/fetch/FetchDataConsumerHandle.h
@@ -7,6 +7,7 @@
 
 #include "modules/ModulesExport.h"
 #include "platform/blob/BlobData.h"
+#include "platform/network/FormData.h"
 #include "public/platform/WebDataConsumerHandle.h"
 #include "wtf/Forward.h"
 #include "wtf/PassRefPtr.h"
@@ -40,6 +41,16 @@ public:
         // function is no-op.
         // This function returns |nullptr| when called during two-phase read.
         virtual PassRefPtr<BlobDataHandle> drainAsBlobDataHandle(BlobSizePolicy = DisallowBlobWithInvalidSize) { return nullptr; }
+
+        // Drains the data as a FormData.
+        // This function returns a non-null form data when
+        // - The handle is made from a FormData-convertible type,
+        // - The reader is running on a suitable thread, and

[hiroshige, 2015/08/11 09:35:01]

What is "a suitable thread"? Isn't this a general threading restriction of the
Reader thread?

[yhirano, 2015/08/11 11:36:01]

Sorry the restriction was gone.

+        // - No data is read yet.
+        // After that read() / beginRead() will return |Done|.

[hiroshige, 2015/08/11 09:35:01]

Please add "and drainAsBlobDataHandle() will return nullptr".

[yhirano, 2015/08/11 11:36:01]

I rewrote comments.

+        // Otherwise this function does nothing and returns null.
+        // This function returns |nullptr| when called during two-phase read.
+        virtual PassRefPtr<FormData> drainAsFormData() { return nullptr; }
     };
 
     // TODO(yhirano): obtainReader() is currently non-virtual override, and