Remove paint/masks/fieldset-mask.html from SlowTests

TBR=fs@opera.com
BUG=529345

Change-Id: If867ae6936ccfadfd99e4cff4ea209c686f99382
Reviewed-on: https://chromium-review.googlesource.com/1249395Reviewed-by: Stephen Chenney <schenney@chromium.org>
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594942}

third_party/WebKit/LayoutTests/SlowTests

@@ -202,8 +202,6 @@ crbug.com/459009 crypto/subtle/ [ Slow ]
 
 crbug.com/528419 http/tests/devtools/elements/styles-2/pseudo-elements.js [ Slow ]
 
-crbug.com/529345 [ Win10 ] paint/masks/fieldset-mask.html [ Slow ]
-
 crbug.com/802029 fast/dom/shadow/focus-controller-recursion-crash.html [ Slow ]
 
 crbug.com/592183 external/wpt/webusb/usbDevice.https.html [ Slow ]