Sync some HTML interfaces with the spec

Source/core/html/TextMetrics.idl

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
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  * modification, are permitted provided that the following conditions
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  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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+
+// https://html.spec.whatwg.org/#textmetrics
+
+// TODO(philipj): All float types in this interface should be double.
 [
+    // TODO(philipj): Exposed=(Window,Worker)
     WillBeGarbageCollected,
 ] interface TextMetrics {
     // x-direction
-    readonly attribute float width;
+    readonly attribute float width; // advance width
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float actualBoundingBoxLeft;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float actualBoundingBoxRight;
 
     // y-direction
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float fontBoundingBoxAscent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float fontBoundingBoxDescent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float actualBoundingBoxAscent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float actualBoundingBoxDescent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float emHeightAscent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float emHeightDescent;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float hangingBaseline;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float alphabeticBaseline;
     [RuntimeEnabled=ExperimentalCanvasFeatures] readonly attribute float ideographicBaseline;
 };