[PE] Change CHECK to LOG(WARNING) in PaintController::FindOutOfOrderCachedItemForward

The situation doesn't cause corrupted rendering but just slightly
affects performance. It's fine not to DCHECK given that the situation
is rare.

Bug: 805024
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Iac8cd617d5df51da2516fc6a9df1308a0daaedd0
Reviewed-on: https://chromium-review.googlesource.com/990074
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547923}

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

@@ -480,7 +480,7 @@ size_t PaintController::FindOutOfOrderCachedItemForward(
 #endif
     // Ensure our paint invalidation tests don't trigger the less performant
     // situation which should be rare.
-    CHECK(false) << "Can't find cached display item: " << id.client.DebugName()
+    LOG(WARNING) << "Can't find cached display item: " << id.client.DebugName()
                  << " " << id.ToString();
   }
   return kNotFound;