Allow preload scanners to be disabled by Settings.

Source/core/frame/Settings.in

 # Defines properties which are available on the Settings object.
 #
 # Please think carefully before adding a new Setting.  Some questions to
 # consider are:
 # - Should this be a RuntimeEnabledFeature instead? Settings are for things
 #   which we support either values of at runtime.  Features are set at renderer
 #   process startup and are never changed. Features also tend to be set to a
 #   value based on the platform or the stability of the code in question, where
 #   as settings both codepaths need to be stable.
 # - How will you ensure test coverage of all relevant values of your setting?
@@ skipping 238 matching lines @@
 # the image URL. A cached image will still be rendered if requested.
 loadsImagesAutomatically initial=false, invalidate=ImageLoading
 imagesEnabled initial=true, invalidate=ImageLoading
 imageAnimationPolicy type=ImageAnimationPolicy, initial=ImageAnimationPolicyAllowed
 
 # Number of outstanding and pending tokens allowed in the background HTML
 # parser. A value of 0 indicates the parser should use its default value.
 backgroundHtmlParserOutstandingTokenLimit type=unsigned, initial=0
 backgroundHtmlParserPendingTokenLimit type=unsigned, initial=0
 
+# Html preload scanning is a fast, early scan of HTML documents to find loadable
+# resources before the parser advances to them. If it is disabled, resources will
+# be loaded later.
+doHtmlPreloadScanning initial=true
+
 javaEnabled initial=false
 pluginsEnabled initial=false
 
 viewportEnabled initial=false, invalidate=ViewportDescription
 viewportMetaEnabled initial=false
 
 dnsPrefetchingEnabled initial=false, invalidate=DNSPrefetching
 
 # FIXME: This is a temporary flag and should be removed
 # when squashing is ready. (crbug.com/261605)
@@ skipping 63 matching lines @@
 # User style overrides for captions and subtitles
 textTrackBackgroundColor type=String
 textTrackFontFamily type=String
 textTrackFontStyle type=String
 textTrackFontVariant type=String
 textTrackTextColor type=String
 textTrackTextShadow type=String
 textTrackTextSize type=String
 
 lowPriorityIframes initial=false