Scroll only visual viewport during OSX pinch gesture behind flag.

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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 touchEditingEnabled initial=false
 
 # Settings for experimental desktop pinch-zoom support (with semantics
 # optimized for large screens).  Pinch-zoom generally is implemented mainly
 # outside of blink (in the compositor) and doesn't require any settings.
 # These settings are for an experimental modification to how pinch-zoom
 # behaves. crbug.com/304869 tracks removal.
 useSolidColorScrollbars initial=false
 pinchOverlayScrollbarThickness type=int, initial=0
 
+# Experiment to invert the scroll order of the layout and visual viewports
+# in some circumstances. Namely, during a pinch-zoom gesture and browser
+# generated scrolls like "scroll into view".
+invertViewportScrollOrder initial=false
+
 # The rubber-band overscroll effect is implemented in Blink and is being moved
 # to the compositor thread. This will be set to true and eventually removed.
 # crbug.com/133097
 rubberBandingOnCompositorThread initial=false
 
 # Font scale factor for accessibility, applied as part of text autosizing.
 accessibilityFontScaleFactor type=double, initial=1.0, invalidate=TextAutosizing
 
 # Only used by Layout Tests and inspector emulation.
 mediaTypeOverride type=String, initial="", invalidate=MediaQuery
@@ skipping 91 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