Guard PaintLayerScrollableArea functions used through ScrollableArea

ScrollAnimator continues to call ShouldUseIntegerScrollOffset and
UpdateScrollOffset indirectly through the base class ScrollableArea.

Bug: 815135
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I495f8ae408022b4e170b17b4392480224cd8114e
Reviewed-on: https://chromium-review.googlesource.com/935041Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539231}

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -298,10 +298,12 @@ GraphicsLayer* PaintLayerScrollableArea::LayerForScrollCorner() const {
 }
 
 bool PaintLayerScrollableArea::ShouldUseIntegerScrollOffset() const {
-  Frame* frame = GetLayoutBox()->GetFrame();
-  if (frame->GetSettings() &&
-      !frame->GetSettings()->GetPreferCompositingToLCDTextEnabled())
-    return true;
+  if (!HasBeenDisposed()) {
+    Frame* frame = GetLayoutBox()->GetFrame();
+    if (frame->GetSettings() &&
+        !frame->GetSettings()->GetPreferCompositingToLCDTextEnabled())
+      return true;
+  }
 
   return ScrollableArea::ShouldUseIntegerScrollOffset();
 }
@@ -430,7 +432,7 @@ int PaintLayerScrollableArea::ScrollSize(
 void PaintLayerScrollableArea::UpdateScrollOffset(
     const ScrollOffset& new_offset,
     ScrollType scroll_type) {
-  if (GetScrollOffset() == new_offset)
+  if (HasBeenDisposed() || GetScrollOffset() == new_offset)
     return;
 
   bool offset_was_zero = scroll_offset_.IsZero();