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REGRESSION (macOS Tahoe): 13x CSSViewportUnits API tests are constant failures
https://bugs.webkit.org/show_bug.cgi?id=294980
rdar://154219736

Reviewed by Simon Fraser.

After rdar://146130865, the width of the system legacy scrollbar on macOS has changed from 15 to 14
(i.e., the width exposed through `-[NSScrollerImp trackBoxWidth]` for a scrollbar with control size
`NSControlSizeRegular`). These API tests currently rely on a hard-coded size of 15, so they began
failing with small deltas.

To fix this, instead of hard-coding a new width here, we adjust the tests to directly ask the system
for the width via `-[NSScrollerImp trackBoxWidth]`.

* Tools/TestWebKitAPI/Tests/WebKitCocoa/CSSViewportUnits.mm:
(scrollbarSize):
(TEST(CSSViewportUnits, AllSame)):
(TEST(CSSViewportUnits, MinimumViewportInset)):
(TEST(CSSViewportUnits, MaximumViewportInset)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithZoom)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithZoom)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithWritingMode)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithWritingMode)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithFrame)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithFrame)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithBounds)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithBounds)):
(TEST(CSSViewportUnits, TopContentInset)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithTopContentInset)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithTopContentInset)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithContentInset)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithContentInset)):
(TEST(CSSViewportUnits, MinimumViewportInsetWithSafeAreaInsets)):
(TEST(CSSViewportUnits, MaximumViewportInsetWithSafeAreaInsets)):
(TEST(CSSViewportUnits, UnobscuredSizeOverridesIgnoreMinimumViewportInset)):
(TEST(CSSViewportUnits, UnobscuredSizeOverridesIgnoreMaximumViewportInset)):
(TEST(CSSViewportUnits, EmptyUnobscuredSizeOverrides)):
(TEST(CSSViewportUnits, SameUnobscuredSizeOverrides)):
(TEST(CSSViewportUnits, DifferentUnobscuredSizeOverrides)):
(TEST(CSSViewportUnits, SVGDocument)):

Canonical link: https://commits.webkit.org/296628@main

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