unified-heap: Add GN flag for full heap snapshots

Blink CL: http://crrev.com/c/1504573

Bug: chromium:936797
Change-Id: I24092f53f40dbd663ffb58122ec0239b7c185904
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1505372Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60048}

gni/v8.gni

@@ -31,6 +31,10 @@ declare_args() {
   # Support for backtrace_symbols on linux.
   v8_enable_backtrace = ""
 
+  # Enables full heap snapshots containing internals. Used for debugging memory
+  # on platform and embedder level.
+  v8_enable_full_heap_snapshots = false
+
   # Enable the snapshot feature, for fast context creation.
   # https://v8.dev/blog/custom-startup-snapshots
   # TODO(thakis): Make snapshots work in 64-bit win/cross builds,