Use is<>() / downcast<>() for File
https://bugs.webkit.org/show_bug.cgi?id=137318

Reviewed by Gyuyoung Kim.

Use is<>() / downcast<>() for File instead of isFile() / toFile().

No new tests, no behavior change.

* fileapi/Blob.h:
* fileapi/File.h:
(isType):
* fileapi/FileReader.cpp:
(WebCore::FileReader::readAsArrayBuffer):
(WebCore::FileReader::readAsBinaryString):
(WebCore::FileReader::readAsText):
(WebCore::FileReader::readAsDataURL):
* platform/network/FormData.cpp:
(WebCore::FormData::appendKeyValuePairItems):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174197 268f45cc-cd09-0410-ab3c-d52691b4dbfc