Fixed WebDragData's wrong dependency.

Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 262 matching lines @@
       'exported/WebConvertableToTraceFormat.cpp',
       'exported/WebCredential.cpp',
       'exported/WebCryptoAlgorithm.cpp',
       'exported/WebCryptoKey.cpp',
       'exported/WebCryptoKeyAlgorithm.cpp',
       'exported/WebCryptoResult.cpp',
       'exported/WebCursorInfo.cpp',
       'exported/WebData.cpp',
       'exported/WebDeviceMotionData.cpp',
       'exported/WebDeviceOrientationData.cpp',
+      'exported/WebDragData.cpp',
       'exported/WebEncryptedMediaClient.cpp',
       'exported/WebEncryptedMediaKeyInformation.cpp',
       'exported/WebFederatedCredential.cpp',
       'exported/WebFileSystemCallbacks.cpp',
       'exported/WebFilterKeyframe.cpp',
       'exported/WebHTTPBody.cpp',
       'exported/WebHTTPLoadInfo.cpp',
       'exported/WebImageSkia.cpp',
       'exported/WebLocalCredential.cpp',
       'exported/WebMediaConstraints.cpp',
@@ skipping 694 matching lines @@
       'graphics/ImageLayerChromiumTest.cpp',
       'graphics/test/MockImageDecoder.h',
       'graphics/test/MockWebGraphicsContext3D.h',
       'image-decoders/bmp/BMPImageDecoderTest.cpp',
       'image-decoders/gif/GIFImageDecoderTest.cpp',
       'image-decoders/jpeg/JPEGImageDecoderTest.cpp',
       'image-decoders/webp/WEBPImageDecoderTest.cpp',
     ],
   },
 }