[oilpan] Avoid write barrier on initilizing stores

Initializing stores (through constructors) can assume that objects are
not yet marked and thus don't require write barriers.

This CL breaks placement new for Member which can violate the assumption
that the enclosing object is white. A follow up will fix this.

Bug: chromium:757440, chromium:790691
Change-Id: Id0ef91ba1887837593603eaf73f56a127f8fda0a
Reviewed-on: https://chromium-review.googlesource.com/812344
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522303}

third_party/WebKit/Source/platform/heap/Member.h

@@ -196,8 +196,12 @@ class Member : public MemberBase<T, TracenessMemberConfiguration::kTraced> {
  public:
   Member() : Parent() {}
   Member(std::nullptr_t) : Parent(nullptr) {}
-  Member(T* raw) : Parent(raw) { WriteBarrier(this->raw_); }
-  Member(T& raw) : Parent(raw) { WriteBarrier(this->raw_); }
+  Member(T* raw) : Parent(raw) {
+    // No write barrier for initializing stores.
+  }
+  Member(T& raw) : Parent(raw) {
+    // No write barrier for initializing stores.
+  }
   Member(WTF::HashTableDeletedValueType x) : Parent(x) {}
 
   Member(const Member& other) : Parent(other) { WriteBarrier(this->raw_); }