Declarative resource hints go through mojo IPC to //content

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 71 matching lines @@
       'PODIntervalTree.h',
       'PODRedBlackTree.h',
       'ParsingUtilities.h',
       'PasteMode.h',
       'PlatformEvent.h',
       'PlatformGestureEvent.h',
       'PlatformInstrumentation.cpp',
       'PlatformInstrumentation.h',
       'PlatformKeyboardEvent.cpp',
       'PlatformKeyboardEvent.h',
+      'PlatformMojoServices.cpp',
+      'PlatformMojoServices.h',
       'PlatformMouseEvent.h',
       'PlatformResourceLoader.cpp',
       'PlatformResourceLoader.h',
       'PlatformTouchEvent.h',
       'PlatformTouchPoint.h',
       'PlatformWheelEvent.h',
       'PluginScriptForbiddenScope.cpp',
       'PluginScriptForbiddenScope.h',
       'PopupMenu.h',
       'Prerender.cpp',
@@ skipping 1233 matching lines @@
       ['use_default_render_theme==1',
         {
           'platform_test_files': [
             'scroll/ScrollbarThemeAuraTest.cpp',
           ],
         }
       ],
     ],
   },
 }