Make scrollAnimatorEnabled work on Mac like it does on other platforms

Mac currently has a separate path to enable/disable scroll animations in
Blink. Instead of using the existing Blink setting for scroll animations,
ScrollbarThemeMacCommon sets a flag scrollAnimationEnabledForSystem which
is then checked along side the Blink setting. A side-effect of this is
that --(disable|enable)-scroll-animator doesn't work on Mac and layout
tests default to scroll animations disabled with no way to turn them on.

This patch removes the scrollAnimationEnabledForSystem flag and instead
unifies this setting with how it's handled on other platforms. When Blink
settings are computed, the OSX setting is queried and set into the usual
enable_scroll_animator setting. Changes to the OSX setting cause the
browser process to recalculate and apply new Blink settings.

This allows layout tests to use internals.settings.setScrollAnimatorEnabled
to allow testing of scroll animation semantics on Mac.

BUG=

Review URL: https://codereview.chromium.org/1653003002

Cr-Commit-Position: refs/heads/master@{#376025}

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