Fix Validation failed for mojom.DiscardableSharedMemoryManager

Make the interface to accept null UnsafeSharedMemoryRegion since both
client and server sides can handle it.

Bug: 1071826
Change-Id: I649a6467b447477f09d8936d1424c07e7adfb495
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159417
Auto-Submit: Peng Huang <penghuang@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761134}

components/discardable_memory/public/mojom/discardable_shared_memory_manager.mojom

@@ -12,7 +12,7 @@ interface DiscardableSharedMemoryManager {
   // Allocate a locked discardable shared memory segment.
   AllocateLockedDiscardableSharedMemory(
       uint32 size,
-      int32 id) => (mojo_base.mojom.UnsafeSharedMemoryRegion region);
+      int32 id) => (mojo_base.mojom.UnsafeSharedMemoryRegion? region);
   // Notify manager that a memory segment has been deleted.
   DeletedDiscardableSharedMemory(int32 id);
 };