[Security] Mark slow web test as such.

[1] was flaky given sporadic timeouts occurring on Win7. This test is
slow as it needs to reach the call stack size limit and then execute
some code in each call frame. This marks the test as Slow which should
give the test enough time to always complete.

Bug: 937546
Change-Id: I450003c435c1a6856c0461c11995ff63f9988de9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2012946
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733933}

third_party/blink/web_tests/SlowTests

@@ -675,3 +675,7 @@ crbug.com/1020379 http/tests/devtools/a11y-axe-core/* [ Slow ]
 
 crbug.com/1040054 http/tests/devtools/cache-storage/cache-data.js [ Slow ]
 crbug.com/1040450 http/tests/devtools/persistence/persistence-external-change-breakpoints.js [ Slow ]
+
+# Reaching the stack size limit makes for a consistently slow test with
+# occasional timeouts on Win7
+crbug.com/937546 [ Win7 ] http/tests/security/xss-DENIED-cross-origin-stack-overflow.html [ Slow ]

third_party/blink/web_tests/TestExpectations

@@ -5737,9 +5737,6 @@ crbug.com/971612 fast/canvas/OffscreenCanvas-MessageChannel-transfer.html [ Pass
 # Sheriff 2019-06-13
 crbug.com/973692 [ Mac ] external/wpt/css/css-text/parsing/hyphens-computed.html [ Failure ]
 crbug.com/973726 [ Mac ] fast/css/large-list-of-rules-crash.html [ Pass Timeout ]
-crbug.com/937546 [ Win7 ] http/tests/security/xss-DENIED-cross-origin-stack-overflow.html [ Pass Timeout ]
-
-# Sheriff 2019-06-14
 
 # Sheriff 2019-06-20
 crbug.com/976045 fast/scrolling/unscrollable-layer-subpixel-size-with-negative-overflow.html [ Pass Failure ]