[css-tables] Make scrollbars visible on mac screenshots

wpt.fyi showed Safari passing these recently-added tests because the
scrollbars don't show in either the tests or their refs.
https://wpt.fyi/css/css-tables/height-distribution/percentage-sizing-of-table-cell-replaced-children-001.html

Aside:
Edge matches Safari's current and Chrome's previous behavior. But
they've confirmed it's a bug:
https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/17505131/
Firefox has a mostly-unrelated incompatibility that doesn't allow us to
check it.

Bug: 837408,838144
Change-Id: I82058d3c22d7668711e9a9a1c977047033d64b46
Reviewed-on: https://chromium-review.googlesource.com/1054770
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569698}

third_party/WebKit/LayoutTests/external/wpt/css/css-tables/height-distribution/percentage-sizing-of-table-cell-children-002-ref.html

@@ -26,6 +26,7 @@
   height: 100%;
 }
 </style>
+<link rel="stylesheet" type="text/css" href="support/scrollbars.css">
 
 <p>The test passes if you see scrollbars but there's no overflow, so you cannot actually scroll.</p>
 

third_party/WebKit/LayoutTests/external/wpt/css/css-tables/height-distribution/percentage-sizing-of-table-cell-children-002.html

@@ -17,7 +17,6 @@
   display: table-cell;
   background: cyan;
   overflow: scroll;
-  margin: 1px 2px 3px 4px;
   padding: 5px 15px 10px 20px;
   border: solid magenta;
   border-width: 12px 9px 6px 3px;
@@ -29,6 +28,7 @@
   height: 100%;
 }
 </style>
+<link rel="stylesheet" type="text/css" href="support/scrollbars.css">
 
 <p>The test passes if you see scrollbars but there's no overflow, so you cannot actually scroll.</p>
 

third_party/WebKit/LayoutTests/external/wpt/css/css-tables/height-distribution/percentage-sizing-of-table-cell-replaced-children-001-ref.html

@@ -27,6 +27,7 @@ img {
   height: 100%;
 }
 </style>
+<link rel="stylesheet" type="text/css" href="support/scrollbars.css">
 
 <p>The test passes if you see scrollbars but there's no overflow, so you cannot actually scroll.</p>
 

third_party/WebKit/LayoutTests/external/wpt/css/css-tables/height-distribution/percentage-sizing-of-table-cell-replaced-children-001.html

@@ -17,7 +17,6 @@
   display: table-cell;
   background: cyan;
   overflow: scroll;
-  margin: 1px 2px 3px 4px;
   padding: 5px 15px 10px 20px;
   border: solid magenta;
   border-width: 12px 9px 6px 3px;
@@ -30,6 +29,7 @@ img {
   height: 100%;
 }
 </style>
+<link rel="stylesheet" type="text/css" href="support/scrollbars.css">
 
 <p>The test passes if you see scrollbars but there's no overflow, so you cannot actually scroll.</p>
 

third_party/WebKit/LayoutTests/external/wpt/css/css-tables/height-distribution/support/scrollbars.css

+/* This makes the scrollbars visible on mac, both to humans and screenshots.*/
+
+::-webkit-scrollbar {
+    -webkit-appearance: none;
+}
+
+::-webkit-scrollbar-track {
+    background-color: #eee;
+    border-radius: 8px;
+}