add external/wpt/webusb/idlharness.https.html to TestExpectations

TBR: reillyg@chromium.org
Bug: 751955
Change-Id: Iac23c3c3df62e4dbccaaf80192297dbbb52a48e8
Reviewed-on: https://chromium-review.googlesource.com/599137Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491657}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3092,3 +3092,4 @@ crbug.com/750310 [ Win7 Debug ] virtual/off-main-thread-fetch/http/tests/inspect
 # Sheriff failures 2017-08-03
 crbug.com/751906 [ Linux Debug ] http/tests/devtools/console/console-correct-suggestions.js [ Timeout Pass ]
 crbug.com/751906 [ Linux Debug ] virtual/mojo-loading/http/tests/devtools/console/console-correct-suggestions.js [ Timeout Pass ]
+crbug.com/751955 external/wpt/webusb/idlharness.https.html [ Timeout Pass ]