Fix WebGLDebugShaders::getTranslatedShaderSource returning null

public/platform/WebGraphicsContext3D.h

Index: public/platform/WebGraphicsContext3D.h
diff --git a/public/platform/WebGraphicsContext3D.h b/public/platform/WebGraphicsContext3D.h
index 668164727b438a78bb5a420f3b72c4849c3ce20e..c0b08c9daed57127efdd8a684824d592fe455b38 100644
--- a/public/platform/WebGraphicsContext3D.h
+++ b/public/platform/WebGraphicsContext3D.h
@@ -391,9 +391,7 @@ public:
     // state is sticky, rather than reported only once.
     virtual WGC3Denum getGraphicsResetStatusARB() { return 0; /* GL_NO_ERROR */ }
 
-    // FIXME: make this function pure virtual once it is implemented in
-    // both command buffer port and in-process port.
-    virtual WebString getTranslatedShaderSourceANGLE(WebGLId shader) { return WebString(); }
+    virtual WebString getTranslatedShaderSourceANGLE(WebGLId shader) = 0;
 
     // GL_CHROMIUM_iosurface
     virtual void texImageIOSurface2DCHROMIUM(WGC3Denum target, WGC3Dint width, WGC3Dint height, WGC3Duint ioSurfaceId, WGC3Duint plane) { }