Mark eye dropper test slow.

The eye dropper test run close to (and sometimes exceed) the 60s
timeout limit on MSAN builds. This is similar with other tests that
are using the color picker[1].

This change marks the test as slow to avoid timeouts on the MSAN
trybots.

[1]: http://crrev.com/c/2330086

Bug: 1134580
Change-Id: Ica5b48dd7690a91c507e5c2517e985599333741b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446194Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#813406}

third_party/blink/web_tests/SlowTests

@@ -540,3 +540,5 @@ crbug.com/1104091 [ Linux ] webcodecs/videoframe-imagebitmap.html [ Slow ]
 crbug.com/1093478 external/wpt/quirks/unitless-length/limited-quirks.html [ Slow ]
 
 crbug.com/1133836 external/wpt/scroll-to-text-fragment/redirects.html [ Slow ]
+
+crbug.com/1134580 virtual/eye-dropper/color-picker-show-eye-dropper.html [ Slow ]