Revert "heap: Trace Vector buffer up to capacity"

This reverts commit 5dba9870.

Reason for revert: quick resolution to regressions in issue 1063593

Original change's description:
> heap: Trace Vector buffer up to capacity
>
> Tracing to capacity means we don't need to check size_ during
> tracing anymore.
>
> Bug: 986235
> Change-Id: Idae046fcbb6a1705c1522d56479d560091415ea4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107983
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Omer Katz <omerkatz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#751353}

TBR=haraken@chromium.org,mlippautz@chromium.org,omerkatz@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 986235
Change-Id: Ib5b207d4f81966fae2f2522bd1c806071ff82c55
Tbr: haraken@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119519
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753321}

third_party/blink/renderer/platform/wtf/vector.h

@@ -628,10 +628,7 @@ class VectorBuffer : protected VectorBufferBase<T, Allocator> {
  public:
   using OffsetRange = typename Base::OffsetRange;
 
-  VectorBuffer() : Base(InlineBuffer(), inlineCapacity) {
-    // Make sure the inline buffer is clear.
-    Base::ClearUnusedSlots(InlineBuffer(), InlineBuffer() + inlineCapacity);
-  }
+  VectorBuffer() : Base(InlineBuffer(), inlineCapacity) {}
 
   VectorBuffer(HashTableDeletedValueType value) : Base(value) {}
   bool IsHashTableDeletedValue() const {
@@ -640,8 +637,6 @@ class VectorBuffer : protected VectorBufferBase<T, Allocator> {
 
   explicit VectorBuffer(wtf_size_t capacity)
       : Base(InlineBuffer(), inlineCapacity) {
-    // Make sure the inline buffer is clear.
-    Base::ClearUnusedSlots(InlineBuffer(), InlineBuffer() + inlineCapacity);
     if (capacity > inlineCapacity)
       Base::AllocateBuffer(capacity);
   }
@@ -2069,7 +2064,7 @@ Vector<T, inlineCapacity, Allocator>::Trace(VisitorDispatcher visitor) const {
       return;
     // Inline buffer requires tracing immediately.
     const T* buffer_begin = buffer;
-    const T* buffer_end = buffer + inlineCapacity;
+    const T* buffer_end = buffer + size();
     if (IsTraceableInCollectionTrait<VectorTraits<T>>::value) {
       for (const T* buffer_entry = buffer_begin; buffer_entry != buffer_end;
            buffer_entry++) {