Reenable external/wpt/css/css-variables/variable-cssText.html

It was disabled to unblock wpt-importer, and the test change has now
been imported and the test should be passing.

Bug: 830270
Change-Id: I5049adc3a5225b3feb7e934bcceeb416dd48ea4b
Reviewed-on: https://chromium-review.googlesource.com/1002813Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549172}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3286,7 +3286,6 @@ crbug.com/791529 external/wpt/css/css-variables/variable-transitions-from-no-val
 crbug.com/791529 external/wpt/css/css-variables/variable-transitions-to-no-value.html [ Skip ]
 crbug.com/791529 external/wpt/css/css-variables/variable-transitions-transition-property-variable-before-value.html [ Skip ]
 crbug.com/791529 external/wpt/css/css-variables/variable-transitions-value-before-transition-property-variable.html [ Skip ]
-crbug.com/830270 external/wpt/css/css-variables/variable-cssText.html [ Skip ]
 
 # Feature Policy changes fullscreen behaviour, tests need updating
 crbug.com/718155 fullscreen/full-screen-iframe-not-allowed.html [ Failure ]