Prefer to use the OES_depth24 extension for WebGL's depth-only back buffers.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h

 /*
  * Copyright (c) 2010, 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 149 matching lines @@
     int sampleCount() const { return m_sampleCount; }
     bool explicitResolveOfMultisampleData() const { return m_antiAliasingMode == MSAAExplicitResolve; }
 
     // Bind to m_drawFramebufferBinding or m_readFramebufferBinding if it's not 0.
     // Otherwise, bind to the default FBO.
     void restoreFramebufferBindings();
 
     void addNewMailboxCallback(PassOwnPtr<Closure> closure) { m_newMailboxCallback = std::move(closure); }
 
 protected: // For unittests
+    struct PLATFORM_EXPORT SupportedExtensions {
+        SupportedExtensions();
+
+        bool multisample;
+        bool packedDepthStencil;
+        bool depth24;
+        bool discardFramebuffer;
+    };
+
     DrawingBuffer(
         PassOwnPtr<WebGraphicsContext3D>,
         PassOwnPtr<Extensions3DUtil>,
-        bool multisampleExtensionSupported,
-        bool packedDepthStencilExtensionSupported,
-        bool discardFramebufferSupported,
+        const SupportedExtensions&,
         PreserveDrawingBuffer,
         WebGraphicsContext3D::Attributes requestedAttributes);
 
     bool initialize(const IntSize&);
 
 private:
     struct TextureParameters {
         DISALLOW_NEW();
         WGC3Denum target;
         WGC3Denum internalColorFormat;
@@ skipping 106 matching lines @@
     Platform3DObject m_drawFramebufferBinding;
     Platform3DObject m_readFramebufferBinding;
     GLenum m_activeTextureUnit;
 
     OwnPtr<WebGraphicsContext3D> m_context;
     OwnPtr<Extensions3DUtil> m_extensionsUtil;
     IntSize m_size;
     WebGraphicsContext3D::Attributes m_requestedAttributes;
     bool m_multisampleExtensionSupported;
     bool m_packedDepthStencilExtensionSupported;
+    bool m_depth24ExtensionSupported;
     bool m_discardFramebufferSupported;
     Platform3DObject m_fbo;
     // DrawingBuffer's output is double-buffered. m_colorBuffer is the back buffer.
     TextureInfo m_colorBuffer;
     struct FrontBufferInfo {
         TextureInfo texInfo;
         WebExternalTextureMailbox mailbox;
     };
     FrontBufferInfo m_frontColorBuffer;
 
@@ skipping 48 matching lines @@
     // used to resize the Canvas.
     SkBitmap m_resizingBitmap;
 
     // Used to flip a bitmap vertically.
     Vector<uint8_t> m_scanline;
 };
 
 } // namespace blink
 
 #endif // DrawingBuffer_h