Don't attempt to provision if no system_network_context_manager

When running unit_tests there is no system_network_context_manager
available, so provisioning can't take place. If that is the case then
don't attempt to provision an origin ID.

Tested this with an Android emulator. Without this change 9 of the
OptimizationGuideHintsManagerTest unit_tests failed, and all pass after
this change applied.

Bug: 1018052
Test: see above
Change-Id: Ic3733014c00fc48d1b6860142f08daffcf3d8391
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881950Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710052}

chrome/browser/media/android/cdm/media_drm_origin_id_manager.cc

@@ -208,18 +208,26 @@ class MediaDrmProvisionHelper {
   MediaDrmProvisionHelper() {
     DVLOG(1) << __func__;
     DCHECK(media::MediaDrmBridge::IsPerOriginProvisioningSupported());
-
-    create_fetcher_cb_ =
-        base::BindRepeating(&content::CreateProvisionFetcher,
-                            g_browser_process->system_network_context_manager()
-                                ->GetSharedURLLoaderFactory());
   }
 
   void Provision(ProvisionedOriginIdCB callback) {
     DVLOG(1) << __func__;
 
+    auto* network_context_manager =
+        g_browser_process->system_network_context_manager();
+    if (!network_context_manager) {
+      // system_network_context_manager() returns nullptr in unit tests.
+      DLOG(WARNING) << "Failed to provision origin ID as no "
+                       "system_network_context_manager";
+      std::move(callback).Run(false, base::nullopt);
+      return;
+    }
+
     complete_callback_ = std::move(callback);
     origin_id_ = base::UnguessableToken::Create();
+    create_fetcher_cb_ = base::BindRepeating(
+        &content::CreateProvisionFetcher,
+        network_context_manager->GetSharedURLLoaderFactory());
 
     // Try provisioning for L3 first.
     media_drm_bridge_ = media::MediaDrmBridge::CreateWithoutSessionSupport(