Remove the allow-displaying-mixed-content setting from Blink.

third_party/WebKit/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 73 matching lines @@
 
 # 3D canvas (WebGL) support.
 webGLEnabled initial=false
 
 webGLErrorsToConsoleEnabled initial=true
 antialiased2dCanvasEnabled initial=true
 antialiasedClips2dCanvasEnabled initial=false
 accelerated2dCanvasMSAASampleCount type=int, initial=0
 
 hyperlinkAuditingEnabled initial=false
-allowDisplayOfInsecureContent initial=true
 allowRunningOfInsecureContent initial=true
 
 mediaControlsOverlayPlayButtonEnabled initial=false
 mediaPlaybackRequiresUserGesture initial=false
 
 presentationRequiresUserGesture initial=true
 
 scrollAnimatorEnabled initial=true
 
 # Used to disable threaded, compositor scrolling for testing purposes.
@@ skipping 306 matching lines @@
 # Variant of the ParseHTMLOnMainThread experiment. This is designed to coalesce
 # TokenizedChunks when the experiment is running in threaded mode.
 parseHTMLOnMainThreadCoalesceChunks initial=false
 
 # Whether the CSSPreloadScanner is used for externally CSS preloads. NoPreload
 # indicates that the scanner will be used, but no preloads issued.
 cssExternalScannerNoPreload initial=false
 cssExternalScannerPreload initial=false
 
 browserSideNavigationEnabled initial=false