Make window.scroll properties relative to the layout viewport by default.

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 222 matching lines @@
 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
 
 # Experiment to have all APIs reflect the layout viewport.
 # crbug.com/489206 tracks the experiment.
-inertVisualViewport initial=false
+inertVisualViewport initial=true
 
 # 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.
@@ skipping 120 matching lines @@
 fetchIncreaseAsyncScriptPriority initial=false
 # Increases the priorities for CSS, Scripts, Fonts and Images all by one level
 # and parser-blocking scripts and visible images by 2.
 # This is used in conjunction with logic on the Chrome side to raise the threshold
 # of "layout-blocking" resources and provide a boost to resources that are needed
 # as soon as possible for something currently on the screen.
 fetchIncreasePriorities initial=false
 
 # The autoplay gesture override experiment is described in crbug.com/487345 .
 autoplayExperimentMode type=String, initial="off"