heap: Add gn flag for V8 Oilpan

The flag allows switching the Blink heap implementation to using V8's
version of Oilpan.

Bug: chromium:1056170
Change-Id: I56e116586b38f4cea0666074d9ecbc8a754d0eeb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504252Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821665}

third_party/blink/public/public_features.gni

@@ -10,6 +10,9 @@ declare_args() {
 
   # Enables young generation collections in Oilpan.
   enable_blink_heap_young_generation = false
+
+  # Enables Blink's heap to use V8's version of Oilpan.
+  enable_blink_heap_use_v8_oilpan = false
 }
 
 declare_args() {

third_party/blink/renderer/platform/heap/BUILD.gn

@@ -16,6 +16,7 @@ buildflag_header("blink_heap_buildflags") {
   flags = [
     "BLINK_HEAP_VERIFICATION=$enable_blink_heap_verification",
     "BLINK_HEAP_YOUNG_GENERATION=$enable_blink_heap_young_generation",
+    "BLINK_HEAP_USE_V8_OILPAN=$enable_blink_heap_use_v8_oilpan",
   ]
 }