Bug fix: SharedMemory's mapped IDs can be empty on OnMemoryDump

On OnMemoryDump, base::SharedMemory's IDs are used to create ownership
edges. However, there is no guarantee that the ID is still valid when
OnMemoryDump is called. Actually, some crashes [1] are found when
MemoryAllocatorDumpGuid::UseSharedMemoryBasedGUIDs() is true [2].

This CL fixes this bug to avoid dumping when shared memory id is
invalid (empty). Also, this CL changes OnMemoryDump use SharedMemory::
mapped_id(), that returns an valid ID only when the shared memory is
actually mapped, so that we can avoid unnecessary dumps.

[1] https://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_rel_ng/builds/501311
[2] This is now always false, but we plan to make this return true to
    use new ownership edges.

Bug: 604726
Change-Id: I2400ba334860e57b9b83a785cd69f4a32b8ef151
Reviewed-on: https://chromium-review.googlesource.com/571386Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Reviewed-by: John Bauman <jbauman@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488134}

components/viz/service/display_embedder/server_shared_bitmap_manager.cc

@@ -144,9 +144,11 @@ bool ServerSharedBitmapManager::OnMemoryDump(
     auto guid = GetSharedBitmapGUIDForTracing(bitmap.first);
     base::UnguessableToken shared_memory_guid;
     if (bitmap.second->memory) {
-      shared_memory_guid = bitmap.second->memory->handle().GetGUID();
-      pmd->CreateSharedMemoryOwnershipEdge(
-          dump->guid(), guid, shared_memory_guid, 0 /* importance*/);
+      shared_memory_guid = bitmap.second->memory->mapped_id();
+      if (!shared_memory_guid.is_empty()) {
+        pmd->CreateSharedMemoryOwnershipEdge(
+            dump->guid(), guid, shared_memory_guid, 0 /* importance*/);
+      }
     } else {
       pmd->CreateSharedGlobalAllocatorDump(guid);
       pmd->AddOwnershipEdge(dump->guid(), guid);

ui/gl/gl_image_shared_memory.cc

@@ -89,9 +89,11 @@ void GLImageSharedMemory::OnMemoryDump(
 
   auto guid =
       gfx::GetSharedMemoryGUIDForTracing(process_tracing_id, shared_memory_id_);
-  auto shared_memory_guid = shared_memory_->handle().GetGUID();
-  pmd->CreateSharedMemoryOwnershipEdge(dump->guid(), guid, shared_memory_guid,
-                                       0 /* importance */);
+  auto shared_memory_guid = shared_memory_->mapped_id();
+  if (!shared_memory_guid.is_empty()) {
+    pmd->CreateSharedMemoryOwnershipEdge(dump->guid(), guid, shared_memory_guid,
+                                         0 /* importance */);
+  }
 }
 
 }  // namespace gl