Enable WebGL rendering into ImageCHROMIUM, backed by a scanout buffer to enable overlay suppor…

Source/platform/graphics/gpu/DrawingBuffer.h

Index: Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.h b/Source/platform/graphics/gpu/DrawingBuffer.h
index b8a51abb68908f13cc491b4e8e54be221b18badb..f50702338777808d19e18aaa7c25d7c357750e6f 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -160,7 +160,7 @@ protected: // For unittests
 private:
     void mailboxReleasedWhileDestructionInProgress(const blink::WebExternalTextureMailbox&);
 
-    unsigned createColorTexture(const IntSize& size = IntSize());
+    unsigned createColorTexture();
     // Create the depth/stencil and multisample buffers, if needed.
     void createSecondaryBuffers();
     bool resizeFramebuffer(const IntSize&);
@@ -204,6 +204,9 @@ private:
     // Helper to texImage2D with pixel==0 case: pixels are initialized to 0.
     // By default, alignment is 4, the OpenGL default setting.
     void texImage2DResourceSafe(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height, GLint border, GLenum format, GLenum type, GLint alignment = 4);
+    // Allocate buffer storage to be sent to compositor using either texImage2D or CHROMIUM_image based on available support.
+    void allocateTextureMemory(Platform3DObject textureId, const IntSize&);
+    void deleteChromiumImageForTexture(Platform3DObject textureId);
 
     PreserveDrawingBuffer m_preserveDrawingBuffer;
     bool m_scissorEnabled;
@@ -262,6 +265,9 @@ private:
     Vector<RefPtr<MailboxInfo> > m_textureMailboxes;
     // Mailboxes that were released by the compositor can be used again by this DrawingBuffer.
     Deque<blink::WebExternalTextureMailbox> m_recycledMailboxQueue;
+    // If we used CHROMIUM_image as the backing storage for our buffers,
+    // we need to know the mapping from texture id to image.
+    HashMap<Platform3DObject, blink::WGC3Duint> m_textureToImageChromiumMap;
 
     RefPtr<ContextEvictionManager> m_contextEvictionManager;