Bug 1599943 - update pixel count expectations and re-enable test_scrollbar_colors.html for linux1804 r=intermittent-reviewers,jmaher

Changes:

Due to scrollbar and other UI element changes in linux1804 this test was permafailing and was marked as such.

Now that migration has completed for mochitest-plain, re-enable the test with updated pixel count expectations.

Differential Revision: https://phabricator.services.mozilla.com/D66647

--HG--
extra : moz-landing-system : lando

widget/tests/mochitest.ini

@@ -13,4 +13,3 @@ skip-if = toolkit != "windows" || e10s # Bug 1267491
 support-files = window_picker_no_crash_child.html
 [test_scrollbar_colors.html]
 skip-if = (os == 'linux' && headless) # bug 1460109
-fail-if = (os == 'linux' && os_version == '18.04') # Bug 1599943

widget/tests/test_scrollbar_colors.html

@@ -55,13 +55,14 @@ const MAC_REFERENCES = [
   ["0,255,255", 1760],
 ];
 
+// Values have been updated from 8100, 720, 1180 for linux1804
 const LINUX_REFERENCES = [
   // Yellow background
-  ["255,255,0", 8100],
+  ["255,255,0", 7744],
   // Blue scrollbar face
-  ["0,0,255", 720],
+  ["0,0,255", 1104],
   // Cyan scrollbar track
-  ["0,255,255", 1180],
+  ["0,255,255", 1152],
 ];
 
 let outer = document.querySelector(".outer");