Add a setting to strictly block "blockable" mixed content.

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?
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 # If true, static text nodes expose inline text box children.
 inlineTextBoxAccessibilityEnabled initial=false
 
 # If true, context menu will be shown on mouse up instead of mouse down.
 # Typically enabled on Windows to match platform convention.
 showContextMenuOnMouseUp initial=false
 
 disableReadingFromCanvas initial=false
 strictMixedContentChecking initial=false
 strictPowerfulFeatureRestrictions initial=false
+strictlyBlockBlockableMixedContent initial=false
 logDnsPrefetchAndPreconnect initial=false
 logPreload initial=false
 
 # If true, the UA styles will include viewportAndroid.css. It is used for
 # Android, and dynamically set by the inspector for mobile emulation.
 useMobileViewportStyle initial=false, invalidate=ViewportRule
 
 # Automatic track selection is performed based on user preference for track kind specified
 # by this setting.
 textTrackKindUserPreference type=TextTrackKindUserPreference, initial=TextTrackKindUserPreference::Default, invalidate=TextTrackKindUserPreference
 
 # 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
 
 reportWheelOverscroll initial=false