Replace many [TreatReturnedNullStringAs=Null] with nullable DOMString

Source/core/html/canvas/WebGLRenderingContextBase.idl

Index: Source/core/html/canvas/WebGLRenderingContextBase.idl
diff --git a/Source/core/html/canvas/WebGLRenderingContextBase.idl b/Source/core/html/canvas/WebGLRenderingContextBase.idl
index 46b68bb0c0288d6fd722b3cc2a4ad42680abf1db..ea6d16b5d7e033acdf594c1ba282cd187569fd60 100644
--- a/Source/core/html/canvas/WebGLRenderingContextBase.idl
+++ b/Source/core/html/canvas/WebGLRenderingContextBase.idl
@@ -561,15 +561,15 @@ typedef unrestricted float GLclampf;
     [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();