[RootLayerScrolls] Shorten test names

The long test names were causing failures on Windows, which
evidently has a too-low limit on file name length.

BUG=808111
TBR=skobes@chromium.org,oprypin@chromium.org
NOTRY=true

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I67181501b11963362e8aac0ae1782e2cfe7c1b4a
Reviewed-on: https://chromium-review.googlesource.com/898062Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533777}

third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2

@@ -993,7 +993,7 @@ crbug.com/644358 paint/invalidation/selection/selection-after-delete.html [ Fail
 crbug.com/644358 paint/invalidation/selection/selection-after-remove.html [ Failure ]
 crbug.com/644358 paint/invalidation/window-resize/window-resize-background-image-fixed-centered-composited.html [ Failure ]
 
-Bug(none) compositing/layer-creation/main-thread-scrolling-for-non-composited-fixed-position-if-overflow-hidden.html [ Failure ]
+Bug(none) compositing/layer-creation/main-thread-scrolling-non-composited-fixed-overflow-hidden.html [ Failure ]
 Bug(none) compositing/layer-creation/main-thread-scrolling-for-non-composited-fixed-position.html [ Failure ]
 Bug(none) fast/dom/vertical-scrollbar-in-rtl.html [ Failure ]
 Bug(none) fast/events/continuous-platform-wheelevent-in-scrolling-div.html [ Failure ]