Define "opaque" FormData objects.

Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 3779 matching lines @@
             'fileapi/FileListTest.cpp',
             'fileapi/FileTest.cpp',
             'frame/ImageBitmapTest.cpp',
             'frame/OriginsUsingFeaturesTest.cpp',
             'frame/RootFrameViewportTest.cpp',
             'frame/SubresourceIntegrityTest.cpp',
             'frame/UseCounterTest.cpp',
             'frame/csp/CSPSourceListTest.cpp',
             'frame/csp/CSPSourceTest.cpp',
             'frame/csp/ContentSecurityPolicyTest.cpp',
+            'html/DOMFormDataTest.cpp',
             'html/HTMLDimensionTest.cpp',
             'html/HTMLFormControlElementTest.cpp',
             'html/HTMLImageElementTest.cpp',
             'html/HTMLInputElementTest.cpp',
             'html/HTMLLinkElementSizesAttributeTest.cpp',
             'html/HTMLSelectElementTest.cpp',
             'html/HTMLTableRowElementTest.cpp',
             'html/HTMLTextFormControlElementTest.cpp',
             'html/LinkRelAttributeTest.cpp',
             'html/TimeRangesTest.cpp',
@@ skipping 42 matching lines @@
             'style/OutlineValueTest.cpp',
             'style/SVGComputedStyleTest.cpp',
             'svg/UnsafeSVGAttributeSanitizationTest.cpp',
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }