Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

Source/core/html/canvas/WebGLRenderingContextBase.idl

 /*
  * Copyright (C) 2009 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 543 matching lines @@
     WebGLContextAttributes getContextAttributes();
 
     GLenum getError();
 
     // object getExtension(DOMString name);
     [Custom] any getExtension(DOMString name);
 
     [Custom] any getFramebufferAttachmentParameter(GLenum target, GLenum attachment, GLenum pname);
     [Custom] any getParameter(GLenum pname);
     [Custom] any getProgramParameter(WebGLProgram? program, GLenum pname);
-    [TreatReturnedNullStringAs=Null] DOMString getProgramInfoLog(WebGLProgram? program);
+    DOMString? getProgramInfoLog(WebGLProgram? program);
     [Custom] any getRenderbufferParameter(GLenum target, GLenum pname);
     [Custom] any getShaderParameter(WebGLShader? shader, GLenum pname);
 
-    [TreatReturnedNullStringAs=Null] DOMString    getShaderInfoLog(WebGLShader? shader);
+    DOMString?   getShaderInfoLog(WebGLShader? shader);
 
     WebGLShaderPrecisionFormat getShaderPrecisionFormat(GLenum shadertype, GLenum precisiontype);
 
-    [TreatReturnedNullStringAs=Null] DOMString    getShaderSource(WebGLShader? shader);
+    DOMString?   getShaderSource(WebGLShader? shader);
 
     [Custom] sequence<DOMString> getSupportedExtensions();
 
     [Custom] any getTexParameter(GLenum target, GLenum pname);
 
     [Custom] any getUniform(WebGLProgram? program, WebGLUniformLocation location);
 
     WebGLUniformLocation getUniformLocation(WebGLProgram? program, DOMString name);
 
     [Custom] any getVertexAttrib(GLuint index, GLenum pname);
@@ skipping 97 matching lines @@
     [Custom] void vertexAttrib2fv(GLuint indx, Float32Array values);
     void vertexAttrib3f(GLuint indx, GLfloat x, GLfloat y, GLfloat z);
     [Custom] void vertexAttrib3fv(GLuint indx, Float32Array values);
     void vertexAttrib4f(GLuint indx, GLfloat x, GLfloat y, GLfloat z, GLfloat w);
     [Custom] void vertexAttrib4fv(GLuint indx, Float32Array values);
     void vertexAttribPointer(GLuint indx, GLint size, GLenum type, GLboolean normalized,
                              GLsizei stride, GLintptr offset);
 
     void viewport(GLint x, GLint y, GLsizei width, GLsizei height);
 };