Add Blink support for showing image placeholders using range requests.

third_party/WebKit/Source/platform/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
 import("//third_party/WebKit/Source/config.gni")
@@ skipping 834 matching lines @@
     "graphics/Path.cpp",
     "graphics/Path.h",
     "graphics/PathTraversalState.cpp",
     "graphics/PathTraversalState.h",
     "graphics/Pattern.cpp",
     "graphics/Pattern.h",
     "graphics/PicturePattern.cpp",
     "graphics/PicturePattern.h",
     "graphics/PictureSnapshot.cpp",
     "graphics/PictureSnapshot.h",
+    "graphics/PlaceholderImage.cpp",
+    "graphics/PlaceholderImage.h",
     "graphics/ProfilingCanvas.cpp",
     "graphics/ProfilingCanvas.h",
     "graphics/RecordingImageBufferSurface.cpp",
     "graphics/RecordingImageBufferSurface.h",
     "graphics/ReplayingCanvas.cpp",
     "graphics/ReplayingCanvas.h",
     "graphics/SquashingDisallowedReasons.cpp",
     "graphics/SquashingDisallowedReasons.h",
     "graphics/StaticBitmapImage.cpp",
     "graphics/StaticBitmapImage.h",
@@ skipping 1111 matching lines @@
     "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
     ":test_support",
     "//cc/surfaces:surface_id",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }