base: Improve DiscardableSharedMemory support for ashmem.

base/memory/discardable_memory_shmem_allocator.cc

Index: base/memory/discardable_memory_shmem_allocator.cc
diff --git a/base/memory/discardable_memory_shmem_allocator.cc b/base/memory/discardable_memory_shmem_allocator.cc
index fdd68673420e84eafe67c5cfc4a90b955eea5714..8abe4569382e8d66f407aba397f296fbd6e17b62 100644
--- a/base/memory/discardable_memory_shmem_allocator.cc
+++ b/base/memory/discardable_memory_shmem_allocator.cc
@@ -18,7 +18,11 @@ class DiscardableMemoryShmemChunkImpl : public DiscardableMemoryShmemChunk {
       : shared_memory_(shared_memory.Pass()) {}
 
   // Overridden from DiscardableMemoryShmemChunk:
-  bool Lock() override { return shared_memory_->Lock(0, 0); }
+  bool Lock() override {
+    auto result = shared_memory_->Lock(0, 0);
+    DCHECK_NE(result, DiscardableSharedMemory::PURGED);
+    return result == DiscardableSharedMemory::SUCCESS;
+  }
   void Unlock() override { shared_memory_->Unlock(0, 0); }
   void* Memory() const override { return shared_memory_->memory(); }
   bool IsMemoryResident() const override {