Keep TextMetrics on the Oilpan heap always.

Source/core/html/TextMetrics.idl

 /*
  * Copyright (C) 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 10 matching lines @@
  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://html.spec.whatwg.org/#textmetrics
 
 // TODO(philipj): All float types in this interface should be double.
 [
     // TODO(philipj): Exposed=(Window,Worker)
-    WillBeGarbageCollected,
+    GarbageCollected,
 ] interface TextMetrics {
     // x-direction
     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;
 };