Add a missing case as a follow up of crrev.com/c/742354

crrev.com/c/742354 broke DisplayItemTest.DebugStringsExist in
blink_platform_unittests. It seems caused by just forgetting to add a case to
DisplayItem.cpp, so this patch is to fix it.

TBR=wangxianzhu@chromium.org, trchen@chromium.org
NOTRY=true

Bug: n/a
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I4faf738cb3fabb91253283396fa46733bf287100
Reviewed-on: https://chromium-review.googlesource.com/748481
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513051}

third_party/WebKit/Source/platform/graphics/paint/DisplayItem.cpp

@@ -90,6 +90,7 @@ static WTF::String SpecialDrawingTypeAsDebugString(DisplayItem::Type type) {
     DEBUG_STRING_CASE(PrintedContentPDFURLRect);
     DEBUG_STRING_CASE(Resizer);
     DEBUG_STRING_CASE(SVGClip);
+    DEBUG_STRING_CASE(SVGClipBoundsHack);
     DEBUG_STRING_CASE(SVGFilter);
     DEBUG_STRING_CASE(SVGMask);
     DEBUG_STRING_CASE(ScrollbarBackButtonEnd);