Make PaintInfo stack allocated

For the transition to garbage collected LayoutObjects,
non-stack-allocated objects need to opt in to tracing [1]. PaintInfo
should be stack allocated and this patch marks it as such. To do this,
DirectionFlippingScope needed to be marked as stack allocated as well.
With this patch, PaintInfo should work as-is during the transition to
garbage collected LayoutObjects.

[1] https://chromium.googlesource.com/chromium/src/+/master/third_party/blink/renderer/platform/heap/BlinkGCAPIReference.md#stack_allocated

Bug: 1030176
Change-Id: Ifddaaf53eedb9f36bfb050cbb1c3edd84daa3780
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633642Reviewed-by: Yuki Yamada <yukiy@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844635}

third_party/blink/renderer/core/paint/paint_info.h

@@ -53,7 +53,7 @@ namespace blink {
 class LayoutBoxModelObject;
 
 struct CORE_EXPORT PaintInfo {
-  USING_FAST_MALLOC(PaintInfo);
+  STACK_ALLOCATED();
 
  public:
   PaintInfo(GraphicsContext& context,

third_party/blink/renderer/core/paint/theme_painter_default.cc

@@ -72,6 +72,8 @@ WebThemeEngine::State GetWebThemeState(const Element& element) {
 }
 
 class DirectionFlippingScope {
+  STACK_ALLOCATED();
+
  public:
   DirectionFlippingScope(const LayoutObject&, const PaintInfo&, const IntRect&);
   ~DirectionFlippingScope();