Restore ShowdowRoot class size in //third_party/blink/renderer/core/dom

The size of ShowdowRoot class had doubled the bitfield size from 16
bits to 32 bits to match the alignment of all compilers after
landing the CL[1]. This CL restores ShowdowRoot class size using
only |unsigned| type with the bitfields.

[1]https://crrev.com/c/1477558/

Bug: 936379
Change-Id: I5faaf4364738835f2fb68036777b148d25bbedc3
Reviewed-on: https://chromium-review.googlesource.com/c/1491743Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636535}

third_party/blink/renderer/core/dom/shadow_root.cc

@@ -57,7 +57,6 @@ void ShadowRoot::Distribute() {
 
 struct SameSizeAsShadowRoot : public DocumentFragment, public TreeScope {
   Member<void*> member[3];
-  uint16_t counter;
   unsigned flags[1];
 };
 

third_party/blink/renderer/core/dom/shadow_root.h

@@ -190,13 +190,13 @@ class CORE_EXPORT ShadowRoot final : public DocumentFragment, public TreeScope {
   TraceWrapperMember<StyleSheetList> style_sheet_list_;
   Member<SlotAssignment> slot_assignment_;
   Member<ShadowRootV0> shadow_root_v0_;
-  uint16_t child_shadow_root_count_;
+  unsigned child_shadow_root_count_ : 16;
   unsigned type_ : 2;
   unsigned registered_with_parent_shadow_root_ : 1;
   unsigned delegates_focus_ : 1;
   unsigned slotting_ : 1;
   unsigned needs_distribution_recalc_ : 1;
-  unsigned unused_ : 26;
+  unsigned unused_ : 10;
 
   DISALLOW_COPY_AND_ASSIGN(ShadowRoot);
 };