IDL: make optional arguments (without default) explicit sometimes

Source/core/fileapi/File.cpp

Index: Source/core/fileapi/File.cpp
diff --git a/Source/core/fileapi/File.cpp b/Source/core/fileapi/File.cpp
index a7d1d80359339c3866edd8099b67c00f7eca0857..5963b41e9e0dd56ca90d5785ea5bffcef8e1e0e9 100644
--- a/Source/core/fileapi/File.cpp
+++ b/Source/core/fileapi/File.cpp
@@ -216,7 +216,7 @@ unsigned long long File::size() const
     return static_cast<unsigned long long>(size);
 }
 
-PassRefPtrWillBeRawPtr<Blob> File::slice(long long start, long long end, const String& contentType, ExceptionState& exceptionState) const
+PassRefPtrWillBeRawPtr<Blob> File::slice(long long start, Optional<long long> optionalEnd, const String& contentType, ExceptionState& exceptionState) const
 {
     if (hasBeenClosed()) {
         exceptionState.throwDOMException(InvalidStateError, "File has been closed.");
@@ -224,7 +224,9 @@ PassRefPtrWillBeRawPtr<Blob> File::slice(long long start, long long end, const S
     }
 
     if (!m_hasBackingFile)
-        return Blob::slice(start, end, contentType, exceptionState);
+        return Blob::slice(start, optionalEnd, contentType, exceptionState);
+
+    long long end = optionalEnd.isMissing() ? std::numeric_limits<long long>::max() : optionalEnd.get();
 
     // FIXME: This involves synchronous file operation. We need to figure out how to make it asynchronous.
     long long size;