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This CL removes lazy load image placeholder logic and tests. It somewhat
accompanies https://crrev.com/c/2125570, and the specific changes are
summarized below:

 - Removes FetchParameters::SetAllowImagePlaceholder, which turns
   eligible image requests into range requests for the first 2k of the
   real image, such as to fetch the placeholder. This was originally
   added in codereview.chromium.org/2423683002.
 - Removes the LAZY_IMAGE_LOAD_DEFERRED previews state, and its
   associated Blink enum value. This was originally added in
   crrev.com/c/1260926, which also stored some lazy loaded image state
   in PlaceHolderImage, and added some placeholder creation logic. This
   CL removes those changes as well.
 - Removes the LAZY_IMAGE_AUTO_RELOAD previews state, and its associated
   Blink enum value. This was originally added in crrev.com/c/1529865,
   which also recorded estimated data savings of Lo-Fi and lazy loaded
   images whose placeholders were first fetched. The Lo-Fi recording
   was removed in crrev.com/c/1652012, and this CL removes the recording
   for lazy loaded images.
 - Removes FetchParameters::ImageRequestOptimization::kAllowPlaceholder,
   and replaces all invocations of CSSImageValue::CacheImage that pass
   in  kAllowPlaceholder, with invocations that pass kNone.
 - [TEST]: Removes TestThatReloadIsStartedThenServeReload, and
   TestThatIsPlaceholderRequestAndServeResponse in the image resource
   tests, and the tests that use them, as they test placeholder/partial
   image fetching, and use FetchParameters::SetAllowImagePlaceholder
   which is going away in this CL.
 - [TEST]: Remove other tests that use
   FetchParameters::SetAllowImagePlaceholder.

Bug: N/A
Change-Id: Ic39036a5a240df483970375a89f35678e0f10866
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137860Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758199}