content: Add valid handle check to...

content: Add valid handle check to ChildDiscardableSharedMemoryManager::AllocateLockedDiscardableSharedMemory.

This check allows us to distinguish between allocation failure
on the browser side and failure to map the memory in the child
process.

BUG=460995

Review URL: https://codereview.chromium.org/939343003

Cr-Commit-Position: refs/heads/master@{#317746}

content/child/child_discardable_shared_memory_manager.cc

@@ -187,6 +187,7 @@ ChildDiscardableSharedMemoryManager::AllocateLockedDiscardableSharedMemory(
   sender_->Send(
       new ChildProcessHostMsg_SyncAllocateLockedDiscardableSharedMemory(
           size, &handle));
+  CHECK(base::SharedMemory::IsHandleValid(handle));
   scoped_ptr<base::DiscardableSharedMemory> memory(
       new base::DiscardableSharedMemory(handle));
   CHECK(memory->Map(size));