[heap] Turn on RO_SPACE sharing for pointer compression

Makes the read-only space sharing the default even with pointer
compression if on Linux, Android or Chrome OS.

This won't have any immediate impact on Chrome since the platform page
allocator in chrome does not yet support allocation and remapping of
shared pages.

Bug: v8:10454
Change-Id: I3bc57080827efe38095a4bb1d02a53518727056a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2454077
Auto-Submit: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70383}

BUILD.gn

@@ -350,7 +350,8 @@ if (v8_multi_arch_build &&
   v8_enable_pointer_compression = !v8_enable_pointer_compression
 }
 if (v8_enable_shared_ro_heap == "") {
-  v8_enable_shared_ro_heap = !v8_enable_pointer_compression
+  v8_enable_shared_ro_heap =
+      !v8_enable_pointer_compression || (is_linux || is_chromeos || is_android)
 }
 
 assert(!v8_disable_write_barriers || v8_enable_single_generation,
@@ -365,7 +366,7 @@ assert(v8_current_cpu == "arm64" || !v8_control_flow_integrity,
 if (v8_enable_shared_ro_heap && v8_enable_pointer_compression) {
   assert(
       is_linux || is_chromeos || is_android,
-      "Sharing read-only heap with pointer compression is only supported on Linux or Android")
+      "Sharing read-only heap with pointer compression is only supported on Linux/Android/ChromeOS")
 }
 
 assert(!v8_use_multi_snapshots || !v8_control_flow_integrity,