Fix WebGLDebugShaders::getTranslatedShaderSource returning null

Source/platform/graphics/test/MockWebGraphicsContext3D.h

Index: Source/platform/graphics/test/MockWebGraphicsContext3D.h
diff --git a/Source/platform/graphics/test/MockWebGraphicsContext3D.h b/Source/platform/graphics/test/MockWebGraphicsContext3D.h
index 162c7a9282ffcbd20a1093d7227a0df42efe6317..cd42b396f3ac631fb3b7d20e1d60f925544c7253 100644
--- a/Source/platform/graphics/test/MockWebGraphicsContext3D.h
+++ b/Source/platform/graphics/test/MockWebGraphicsContext3D.h
@@ -284,6 +284,8 @@ public:
     virtual void getQueryivEXT(GLenum, GLenum, GLint*) { }
     virtual void getQueryObjectuivEXT(WebGLId, GLenum, GLuint*) { }
 
+    virtual WebString getTranslatedShaderSourceANGLE(WebGLId) { return WebString(); }
+
 protected:
     unsigned m_nextTextureId;
     Attributes m_attrs;