Make the background-color of the <img> element light gray

Implement ingenious idea for image document.

Bug: 844743
Change-Id: I6fa9b373999a77e1a8eb8fd399b8f0e59834063c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2455448
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815588}

third_party/blink/renderer/core/html/image_document.cc

@@ -422,6 +422,15 @@ void ImageDocument::UpdateImageStyle() {
   else if (cursor_mode == kZoomOut)
     image_style.Append("cursor: zoom-out;");
 
+  if (GetFrame()->IsMainFrame()) {
+    if (image_is_loaded_) {
+      image_style.Append("background-color: hsl(0, 0%, 90%);");
+      image_style.Append("transition: background-color 300ms;");
+    } else if (image_size_is_known_) {
+      image_style.Append("background-color: hsl(0, 0%, 25%);");
+    }
+  }
+
   image_element_->setAttribute(html_names::kStyleAttr,
                                image_style.ToAtomicString());
 }
@@ -437,6 +446,7 @@ void ImageDocument::ImageUpdated() {
     return;
 
   image_size_is_known_ = true;
+  UpdateImageStyle();
 
   if (ShouldShrinkToFit()) {
     // Force resizing of the image

third_party/blink/web_tests/http/tests/security/contentSecurityPolicy/image-document-default-src-none-expected.txt

@@ -4,4 +4,6 @@ CONSOLE ERROR: Refused to apply inline style because it violates the following C
 
 CONSOLE ERROR: Refused to apply inline style because it violates the following Content Security Policy directive: "default-src 'none'". Either the 'unsafe-inline' keyword, a hash ('sha256-VPZ2mdsWWlqXOFgt1tAllbbJhG8t9bh6emP1o9GwJxY='), or a nonce ('nonce-...') is required to enable inline execution. Note also that 'style-src' was not explicitly set, so 'default-src' is used as a fallback.
 
+CONSOLE ERROR: Refused to apply inline style because it violates the following Content Security Policy directive: "default-src 'none'". Either the 'unsafe-inline' keyword, a hash ('sha256-VPZ2mdsWWlqXOFgt1tAllbbJhG8t9bh6emP1o9GwJxY='), or a nonce ('nonce-...') is required to enable inline execution. Note also that 'style-src' was not explicitly set, so 'default-src' is used as a fallback.
+
 Ensure that we don't crash when loading an ImageDocument that sets CSP headers 

third_party/blink/web_tests/images/huge-image-viewport-scale-expected.txt

@@ -4,5 +4,5 @@ Passes if the image shinks to fit the width.
 
 
 viewport meta: <meta name="viewport" content="width=device-width, minimum-scale=0.1">
-image style: -webkit-user-select: none;margin: auto;cursor: zoom-in;
+image style: -webkit-user-select: none;margin: auto;cursor: zoom-in;background-color: hsl(0, 0%, 90%);transition: background-color 300ms;
 image actual width: 100