[fuchsia] Fix deletion threading for sandbox policy's ServiceDirectory.

SandboxPolicyFuchsia is not thread affine but its ServiceDirectory
is not. This CL schedules the ServiceDirectory's deletion on the
same thread that was used for its construction.

Change-Id: I98e950481e49106ba64c759cef2c903b5dcf5d3b
Reviewed-on: https://chromium-review.googlesource.com/1171673Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582457}

content/common/sandbox_policy_fuchsia.cc

@@ -16,6 +16,7 @@
 #include "base/fuchsia/filtered_service_directory.h"
 #include "base/process/launch.h"
 #include "base/process/process.h"
+#include "base/threading/thread_task_runner_handle.h"
 #include "content/public/common/content_switches.h"
 #include "services/service_manager/sandbox/switches.h"
 
@@ -25,7 +26,11 @@ constexpr const char* const kRendererServices[] = {
     fuchsia::fonts::FontProvider::Name_};
 
 SandboxPolicyFuchsia::SandboxPolicyFuchsia() = default;
-SandboxPolicyFuchsia::~SandboxPolicyFuchsia() = default;
+
+SandboxPolicyFuchsia::~SandboxPolicyFuchsia() {
+  service_directory_task_runner_->DeleteSoon(FROM_HERE,
+                                             std::move(service_directory_));
+}
 
 void SandboxPolicyFuchsia::Initialize(service_manager::SandboxType type) {
   DCHECK_NE(type, service_manager::SANDBOX_TYPE_INVALID);
@@ -39,6 +44,7 @@ void SandboxPolicyFuchsia::Initialize(service_manager::SandboxType type) {
   }
 
   if (type_ == service_manager::SANDBOX_TYPE_RENDERER) {
+    service_directory_task_runner_ = base::ThreadTaskRunnerHandle::Get();
     // Create FilteredServicesDirectory for the renderer process and export all
     // services in kRendererServices. ServiceDirectoryProxy must be initialized
     // on a thread that has async_dispatcher.

content/common/sandbox_policy_fuchsia.h

@@ -5,10 +5,12 @@
 #ifndef CONTENT_COMMON_SANDBOX_POLICY_FUCHSIA_H_
 #define CONTENT_COMMON_SANDBOX_POLICY_FUCHSIA_H_
 
+#include "base/memory/ref_counted.h"
 #include "services/service_manager/sandbox/sandbox_type.h"
 
 namespace base {
 struct LaunchOptions;
+class SequencedTaskRunner;
 
 namespace fuchsia {
 class FilteredServiceDirectory;
@@ -38,6 +40,7 @@ class SandboxPolicyFuchsia {
 
   // Services directory used for the /svc namespace of the child process.
   std::unique_ptr<base::fuchsia::FilteredServiceDirectory> service_directory_;
+  scoped_refptr<base::SequencedTaskRunner> service_directory_task_runner_;
 };
 
 }  // namespace content