Mark layout testhttp/tests/devtools/layers/layer-scroll-rects-get.js as flaky

TBR=bokan@chromium.org
NOTRY=true

Bug: 859169
Change-Id: I36dfab32c886820aa2a5416c9e4984b9fce0f110
Reviewed-on: https://chromium-review.googlesource.com/1121080Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571610}

third_party/WebKit/LayoutTests/TestExpectations

@@ -4733,3 +4733,4 @@ crbug.com/857520 [ Mac Win ] external/wpt/accelerometer/Accelerometer-iframe-acc
 
 # Sheriff 2018-06-29
 crbug.com/859064 [ Mac10.13 ] http/tests/devtools/console-resource-errors.js [ Failure ]
+crbug.com/859169 [ Win7 ] http/tests/devtools/layers/layer-scroll-rects-get.js [ Failure Pass ]