Keep TextMetrics on the Oilpan heap always.

Source/modules/canvas2d/CanvasRenderingContext2D.h

 /*
  * Copyright (C) 2006, 2007, 2009, 2010, 2011, 2012 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 179 matching lines @@
     String textBaseline() const;
     void setTextBaseline(const String&);
 
     String direction() const;
     void setDirection(const String&);
 
     void fillText(const String& text, float x, float y);
     void fillText(const String& text, float x, float y, float maxWidth);
     void strokeText(const String& text, float x, float y);
     void strokeText(const String& text, float x, float y, float maxWidth);
-    PassRefPtrWillBeRawPtr<TextMetrics> measureText(const String& text);
+    TextMetrics* measureText(const String& text);
 
     bool imageSmoothingEnabled() const;
     void setImageSmoothingEnabled(bool);
 
     void getContextAttributes(Canvas2DContextAttributes&) const;
 
     void drawFocusIfNeeded(Element*);
     void drawFocusIfNeeded(Path2D*, Element*);
 
     void addHitRegion(const HitRegionOptions&, ExceptionState&);
@@ skipping 105 matching lines @@
     HashMap<String, Font> m_fontsResolvedUsingCurrentStyle;
     bool m_pruneLocalFontCacheScheduled;
     ListHashSet<String> m_fontLRUList;
 };
 
 DEFINE_TYPE_CASTS(CanvasRenderingContext2D, CanvasRenderingContext, context, context->is2d(), context.is2d());
 
 } // namespace blink
 
 #endif // CanvasRenderingContext2D_h