external/wpt/requestidlecallback/callback-idle-periods.html flakily times out in Debug

The test was previously marked as timing out on Windows. However:
- It does occasionally pass.
- It doesn't actually time out on Windows Release.
- It does time out in Debug builds on other platforms though.

TBR=pdr@chromium.org

Bug: 666993, 898456
Change-Id: I5071af34d4fdb8bed911e864b28ce81eb954fff0
Reviewed-on: https://chromium-review.googlesource.com/c/1297146Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602339}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3973,7 +3973,7 @@ crbug.com/888470 virtual/outofblink-cors-ns/external/wpt/referrer-policy/css-int
 # 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 [ Win ] external/wpt/requestidlecallback/callback-idle-periods.html [ Timeout ]
+crbug.com/666993 [ Debug ] external/wpt/requestidlecallback/callback-idle-periods.html [ Pass 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 ]