[Fuchsia] Fix crash in ~SandboxPolicyFuchsia()

The function as crashing browser process after GPU process is crashed.

Bug: 874533
Change-Id: I2c506b6f90642a0881aa510807cb407e1b655050
Reviewed-on: https://chromium-review.googlesource.com/1178753Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583829}

content/common/sandbox_policy_fuchsia.cc

@@ -28,8 +28,10 @@ constexpr const char* const kRendererServices[] = {
 SandboxPolicyFuchsia::SandboxPolicyFuchsia() = default;
 
 SandboxPolicyFuchsia::~SandboxPolicyFuchsia() {
+  if (service_directory_) {
     service_directory_task_runner_->DeleteSoon(FROM_HERE,
                                                std::move(service_directory_));
+  }
 }
 
 void SandboxPolicyFuchsia::Initialize(service_manager::SandboxType type) {