Support the DPR header for client hints.

Source/core/rendering/RenderImage.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011-2012. All rights reserved.
  *
@@ skipping 146 matching lines @@
 
     if (hasBoxDecorations() || hasMask())
         RenderReplaced::imageChanged(newImage, rect);
 
     if (!m_imageResource)
         return;
 
     if (newImage != m_imageResource->imagePtr())
         return;
 
+    // Per the spec, we let the server-sent header override srcset/other sources of dpr.
+    // https://github.com/igrigorik/http-client-hints/blob/master/draft-grigorik-http-client-hints-01.txt#L255
+    if (m_imageResource->cachedImage() && m_imageResource->cachedImage()->hasDevicePixelRatioHeaderValue())
+        m_imageDevicePixelRatio = 1 / m_imageResource->cachedImage()->devicePixelRatioHeaderValue();
+
     if (!m_didIncrementVisuallyNonEmptyPixelCount) {
         // At a zoom level of 1 the image is guaranteed to have an integer size.
         view()->frameView()->incrementVisuallyNonEmptyPixelCount(flooredIntSize(m_imageResource->imageSize(1.0f)));
         m_didIncrementVisuallyNonEmptyPixelCount = true;
     }
 
     bool imageSizeChanged = false;
 
     // Set image dimensions, taking into account the size of the alt text.
     if (m_imageResource->errorOccurred() || !newImage) {
@@ skipping 427 matching lines @@
         return 0;
 
     ImageResource* cachedImage = m_imageResource->cachedImage();
     if (cachedImage && cachedImage->image() && cachedImage->image()->isSVGImage())
         return toSVGImage(cachedImage->image())->embeddedContentBox();
 
     return 0;
 }
 
 } // namespace WebCore