Chrome Version: M70
OS: Mac

What steps will reproduce the problem?
(1) ensure Mac using show scrollbars when scrolling settings 
(2) open a page has scrollbar, scroll to show the scrollbar
(3) move mouse to scrollbar, wait a while

What is the expected result?

scrollbar should not fade out. As Safari

What happens instead?

scrollbar fade out.

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I tried to call SetScrollbarsVisibleForTesting(true); in ScrollAnimatorMac::MouseEnteredScrollbar. But it does not work.

SetScrollbarsVisibleForTesting calls timer stop, but it seems flashScrollers will cause system callback to start a animation.

#0	0x000000014b11cff7 in blink::BlinkScrollbarPartAnimationTimer::Start() at /Users/chaopeng/workspace/chromium/src/out/Default/../../third_party/blink/renderer/core/scroll/scroll_animator_mac.mm:316
#1	0x000000014b11cefc in ::-[BlinkScrollbarPartAnimation startAnimation]() at /Users/chaopeng/workspace/chromium/src/out/Default/../../third_party/blink/renderer/core/scroll/scroll_animator_mac.mm:390
#2	0x000000014b11e19b in ::-[BlinkScrollbarPainterDelegate setUpAlphaAnimation:scrollerPainter:part:animateAlphaTo:duration:](WTF::RetainPtr<BlinkScrollbarPartAnimation> &, ScrollbarPainter, blink::ScrollbarPart, CGFloat, NSTimeInterval) at /Users/chaopeng/workspace/chromium/src/out/Default/../../third_party/blink/renderer/core/scroll/scroll_animator_mac.mm:549
#3	0x000000014b11e611 in ::-[BlinkScrollbarPainterDelegate scrollerImp:animateKnobAlphaTo:duration:](id, CGFloat, NSTimeInterval) at /Users/chaopeng/workspace/chromium/src/out/Default/../../third_party/blink/renderer/core/scroll/scroll_animator_mac.mm:561
#4	0x00007fff4f3e6ab4 in -[NSScrollerImp setOverlayScrollerState:forceImmediately:] ()