Mark page-frames.js inspector layout test flaky

https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=webkit_layout_tests&tests=inspector-protocol/timeline/page-frames.js
shows that page-frames.js is flaky.

Bug: 734762
Change-Id: Ib16216060b2139b8e1d7ef7aec7b3c5a28c9140a
Reviewed-on: https://chromium-review.googlesource.com/1135670Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Peter Mayo <petermayo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575181}

third_party/WebKit/LayoutTests/TestExpectations

@@ -3863,7 +3863,7 @@ crbug.com/737959 http/tests/misc/object-image-load-outlives-gc-without-crashing.
 
 crbug.com/737959 http/tests/misc/video-poster-image-load-outlives-gc-without-crashing.html [ Failure Pass Crash ]
 
-crbug.com/734762 inspector-protocol/timeline/page-frames.js [ Failure ]
+crbug.com/734762 inspector-protocol/timeline/page-frames.js [ Failure Pass ]
 
 crbug.com/736255 [ Mac ] external/wpt/css/vendor-imports/mozilla/mozilla-central-reftests/flexbox/flexbox-baseline-multi-item-vert-001b.html [ Skip ]