Fix WebGLDebugShaders::getTranslatedShaderSource returning null

public/platform/WebGraphicsContext3D.h

 /*
  * Copyright (C) 2012 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 373 matching lines @@
 
     virtual void setContextLostCallback(WebGraphicsContextLostCallback* callback) { }
     virtual void setErrorMessageCallback(WebGraphicsErrorMessageCallback* callback) { }
     // GL_ARB_robustness
     //
     // This entry point must provide slightly different semantics than
     // the GL_ARB_robustness extension; specifically, the lost context
     // 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) { }
 
     // GL_EXT_texture_storage
     virtual void texStorage2DEXT(WGC3Denum target, WGC3Dint levels, WGC3Duint internalformat,
                                  WGC3Dint width, WGC3Dint height) { }
 
     // GL_EXT_occlusion_query
     virtual WebGLId createQueryEXT() { return 0; }
@@ skipping 62 matching lines @@
     virtual void vertexAttribDivisorANGLE(WGC3Duint index, WGC3Duint divisor) { }
 
     // GL_EXT_multisampled_render_to_texture
     virtual void framebufferTexture2DMultisampleEXT(WGC3Denum target, WGC3Denum attachment, WGC3Denum textarget, WebGLId texture, WGC3Dint level, WGC3Dsizei samples) { }
     virtual void renderbufferStorageMultisampleEXT(WGC3Denum target, WGC3Dsizei samples, WGC3Denum internalformat, WGC3Dsizei width, WGC3Dsizei height) { };
 };
 
 } // namespace blink
 
 #endif