Do not DCHECK for empty encryption key.

On macOS, it's possible if the keychain is locked for the key
to be empty. Platforms that expect a key to never be empty
already DCHECK inside os_crypt.

This restores previous functionality in 113d3089 and prior.

BUG=1016816

Change-Id: Ie407bdb3804804334c28bbebb3775767e674f49a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876734Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Eric Roman <eroman@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708717}

chrome/browser/net/system_network_context_manager.cc

@@ -676,9 +676,8 @@ void SystemNetworkContextManager::OnNetworkServiceCreated(
   // The OSCrypt keys are process bound, so if network service is out of
   // process, send it the required key.
   if (content::IsOutOfProcessNetworkService()) {
-    std::string key = OSCrypt::GetRawEncryptionKey();
-    DCHECK(!key.empty());
-    content::GetNetworkService()->SetEncryptionKey(key);
+    content::GetNetworkService()->SetEncryptionKey(
+        OSCrypt::GetRawEncryptionKey());
   }
 #endif