[heap] Store start of LinearAllocationArea

Right now the allocation observer counters are increased by the LAB size
when allocating the LAB and are decreased later when retiring the rest
of the LAB. In the future counters will be monotonic by increasing them
by the actual used size when retiring a LAB. The start of the LAB is
required for that.

Bug: v8:10315
Change-Id: Ibeca34914f61abcea2a1c1a7b9a042c9be14a589
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2316100Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69039}

src/heap/spaces.h

@@ -369,14 +369,19 @@ class PageRange {
 // space.
 class LinearAllocationArea {
  public:
-  LinearAllocationArea() : top_(kNullAddress), limit_(kNullAddress) {}
-  LinearAllocationArea(Address top, Address limit) : top_(top), limit_(limit) {}
+  LinearAllocationArea()
+      : start_(kNullAddress), top_(kNullAddress), limit_(kNullAddress) {}
+  LinearAllocationArea(Address top, Address limit)
+      : start_(top), top_(top), limit_(limit) {}
 
   void Reset(Address top, Address limit) {
+    start_ = top;
     set_top(top);
     set_limit(limit);
   }
 
+  V8_INLINE Address start() const { return start_; }
+
   V8_INLINE void set_top(Address top) {
     SLOW_DCHECK(top == kNullAddress || (top & kHeapObjectTagMask) == 0);
     top_ = top;
@@ -404,6 +409,8 @@ class LinearAllocationArea {
 #endif
 
  private:
+  // Current allocation top.
+  Address start_;
   // Current allocation top.
   Address top_;
   // Current allocation limit.