Update feature policy unoptimized-images policy

After research, tentatively updating the unoptimized-images policy as:
no using more than 0.5 byte per pixel of image data with approximate
header size (1KB) removed.

Bug: 901925
Change-Id: Ibc5b2e2e6ccaba7a273c63440d48d4e0d154cc2e
Reviewed-on: https://chromium-review.googlesource.com/c/1318372Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Paul Meyer <paulmeyer@chromium.org>
Commit-Queue: Luna Lu <loonybear@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605677}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5475,9 +5475,6 @@ crbug.com/898378 [ Mac10.13 ] virtual/scroll_customization/fast/scroll-behavior/
 crbug.com/889185 [ Win10 ] http/tests/images/feature-policy-image-policies-with-border-radius.html [ Failure ]
 crbug.com/889185 [ Win10 ] http/tests/images/feature-policy-legacy-formats.html [ Failure ]
 
-# Sheriff 2018-10-25
-crbug.com/874576 http/tests/images/feature-policy-unoptimized-images-cached-image.html [ Skip ]
-
 # Sheriff 2018-10-26
 ### See crbug.com/891427 comment near the top of this file:
 ####crbug.com/899087 [ Linux ] virtual/android/fullscreen/full-screen-iframe-allowed-video.html [ Failure Pass ]

third_party/blink/renderer/core/loader/resource/image_resource_content.cc

@@ -501,8 +501,8 @@ bool ImageResourceContent::IsAcceptableContentType() {
 }
 
 // Return true if the image content is well-compressed (and not full of
-// extraneous metadata). This is currently defined as no using more than 10 bits
-// per pixel of image data.
+// extraneous metadata). This is currently defined as no using more than 0.5
+// byte per pixel of image data with approximate header size(1KB) removed.
 // TODO(crbug.com/838263): Support site-defined bit-per-pixel ratio through
 // feature policy declarations.
 bool ImageResourceContent::IsAcceptableCompressionRatio() {
@@ -512,7 +512,7 @@ bool ImageResourceContent::IsAcceptableCompressionRatio() {
   DCHECK(image_);
   double resource_length = image_->Data() ? image_->Data()->size() : 0;
   // Allow no more than 10 bits per compressed pixel
-  return resource_length / pixels <= 1.25;
+  return (resource_length - 1024) / pixels <= 0.5;
 }
 
 void ImageResourceContent::DecodedSizeChangedTo(const blink::Image* image,