[heap] Shrink RO_SPACE pages for Immortal immovable objects

Shrink RO_SPACE since it contains Immortal immovable objects and is
otherwise reporting a virtual size of 256KB when only half of that
will ever be used.

Bug: v8:9230, v8:7464
Change-Id: I68c17bb6c4ff12170774bad6a07dbb8b9d49cce1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906207
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64865}

src/heap/read-only-heap.cc

@@ -97,6 +97,7 @@ ReadOnlyHeap* ReadOnlyHeap::CreateAndAttachToIsolate(Isolate* isolate) {
 
 void ReadOnlyHeap::InitFromIsolate(Isolate* isolate) {
   DCHECK(!init_complete_);
+  read_only_space_->ShrinkImmortalImmovablePages();
 #ifdef V8_SHARED_RO_HEAP
   void* const isolate_ro_roots = reinterpret_cast<void*>(
       isolate->roots_table().read_only_roots_begin().address());