Make length property return useful values for DOM bindings functions

Source/core/html/canvas/DataView.idl

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 [
-    CustomConstructor,
+    CustomConstructor(ArrayBuffer buffer, optional unsigned long byteOffset, optional unsigned long byteLength),
     CustomToJSObject
 ] interface DataView : ArrayBufferView {
     // All these methods raise an exception if they would read or write beyond the end of the view.
 
     // We have to use custom code because our code generator does not support int8_t type.
     // int8_t getInt8(unsigned long byteOffset);
     // uint8_t getUint8(unsigned long byteOffset);
     [Custom, RaisesException] any getInt8();
     [Custom, RaisesException] any getUint8();
 
@@ skipping 13 matching lines @@
     [Custom, RaisesException] void setUint8();
 
     [StrictTypeChecking, RaisesException] void setInt16(unsigned long byteOffset, short value, optional boolean littleEndian);
     [StrictTypeChecking, RaisesException] void setUint16(unsigned long byteOffset, unsigned short value, optional boolean littleEndian);
     [StrictTypeChecking, RaisesException] void setInt32(unsigned long byteOffset, long value, optional boolean littleEndian);
     [StrictTypeChecking, RaisesException] void setUint32(unsigned long byteOffset, unsigned long value, optional boolean littleEndian);
     [StrictTypeChecking, RaisesException] void setFloat32(unsigned long byteOffset, float value, optional boolean littleEndian);
     [StrictTypeChecking, RaisesException] void setFloat64(unsigned long byteOffset, double value, optional boolean littleEndian);
 };