[api] Add V8::SetIsCrossOriginIsolated method

- Adds a method to tell V8 that the process is cross-origin-isolated
  under COOP+COEP. In this case, SharedArrayBuffer can be enabled.

Bug: chromium:923807
Change-Id: I729093665a50d2b3667c028e05b42d21d76b12d3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2502448
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70805}

include/v8.h

@@ -9832,6 +9832,12 @@ class V8_EXPORT V8 {
    */
   static void GetSharedMemoryStatistics(SharedMemoryStatistics* statistics);
 
+  /**
+   * Notifies V8 that the process is cross-origin-isolated, which enables
+   * defining the SharedArrayBuffer function on the global object of Contexts.
+   */
+  static void SetIsCrossOriginIsolated();
+
  private:
   V8();
 

src/api/api.cc

@@ -5862,6 +5862,10 @@ void V8::GetSharedMemoryStatistics(SharedMemoryStatistics* statistics) {
   i::ReadOnlyHeap::PopulateReadOnlySpaceStatistics(statistics);
 }
 
+void V8::SetIsCrossOriginIsolated() {
+  i::FLAG_harmony_sharedarraybuffer = true;
+}
+
 template <typename ObjectType>
 struct InvokeBootstrapper;