Guard some context_ dereferences in ServiceWorkerVersion.

As a WeakPtr, context_ may be null.

BUG=566935

Review URL: https://codereview.chromium.org/1508573002

Cr-Commit-Position: refs/heads/master@{#363428}
(cherry picked from commit aefd501)

Review URL: https://codereview.chromium.org/1515013003 .

Cr-Commit-Position: refs/branch-heads/2564@{crosswalk-project#325}
Cr-Branched-From: 1283eca-refs/heads/master@{#359700}

content/browser/service_worker/service_worker_version.cc

@@ -1769,7 +1769,7 @@ void ServiceWorkerVersion::DidNavigateClient(int request_id,
                                              int render_frame_id) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
 
-  if (running_status() != RUNNING)
+  if (!context_ || running_status() != RUNNING)
     return;
 
   if (render_process_id == ChildProcessHost::kInvalidUniqueID &&
@@ -1961,7 +1961,7 @@ void ServiceWorkerVersion::GetWindowClients(
   if (!options.include_uncontrolled) {
     for (auto& controllee : controllee_map_)
       AddWindowClient(controllee.second, &clients_info);
-  } else {
+  } else if (context_) {
     for (auto it =
              context_->GetClientProviderHostIterator(script_url_.GetOrigin());
          !it->IsAtEnd(); it->Advance()) {
@@ -2000,7 +2000,7 @@ void ServiceWorkerVersion::GetNonWindowClients(
     for (auto& controllee : controllee_map_) {
       AddNonWindowClient(controllee.second, options, clients);
     }
-  } else {
+  } else if (context_) {
     for (auto it =
              context_->GetClientProviderHostIterator(script_url_.GetOrigin());
          !it->IsAtEnd(); it->Advance()) {
@@ -2049,6 +2049,9 @@ void ServiceWorkerVersion::OnTimeoutTimer() {
          running_status() == STOPPING)
       << running_status();
 
+  if (!context_)
+    return;
+
   MarkIfStale();
 
   // Stopping the worker hasn't finished within a certain period.