Mark callback-idle-periods Timeout Win only

This test only times out on Windows Debug.  It runs relatively
quickly and passes on the other bots.  It may not be useful,
but this eliminates noise of incorrect passing, and if it is
(correctly) made to report a failure the expectation can
be updated accurately simultaneously.

Bug: 666993
Change-Id: I24e2da435f964c3fe7de7f91a6949885f713f361
Reviewed-on: https://chromium-review.googlesource.com/1134030
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575183}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3465,7 +3465,7 @@ crbug.com/796668 external/wpt/longtask-timing/longtask-in-sibling-iframe-crossor
 # This test times out on debug builds, see https://crbug.com/755810
 crbug.com/626703 [ Debug ] external/wpt/html/semantics/tabular-data/processing-model-1/span-limits.html [ Skip ]
 
-crbug.com/666993 external/wpt/requestidlecallback/callback-idle-periods.html [ Timeout ]
+crbug.com/666993 [ Win ] external/wpt/requestidlecallback/callback-idle-periods.html [ Timeout ]
 
 # Crashes with DCHECK enabled, but not on normal Release builds.
 crbug.com/809935 external/wpt/css/css-fonts/variations/font-style-interpolation.html [ Timeout ]