Remove GraphicsLayer::GetLayerTreeAsTextForTesting()

This appears to be unused, undefined, and obsolete.

Change-Id: If5e431f77db6c0392662ff76cee2adcd856c07ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865809Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706974}

third_party/blink/renderer/platform/graphics/graphics_layer.h

@@ -199,11 +199,6 @@ class PLATFORM_EXPORT GraphicsLayer : public cc::LayerClient,
   // For hosting this GraphicsLayer in a native layer hierarchy.
   cc::PictureLayer* CcLayer() const;
 
-  // Return a string with a human readable form of the layer tree. If debug is
-  // true, pointers for the layers and timing data will be included in the
-  // returned string.
-  String GetLayerTreeAsTextForTesting(LayerTreeFlags = kLayerTreeNormal) const;
-
   void UpdateTrackingRasterInvalidations();
   void ResetTrackedRasterInvalidations();
   bool HasTrackedRasterInvalidations() const;