[OSCrypt] Remove DCHECK on key not being empty on mac

Subprocesses set their encryption key by getting it from the browser with
GetRawEncryptionKey and setting it locally with SetRawEncryptionKey

Since GetRawEncryptionKey may return an empty string (e.g. because the
lookup into Keychain failed), SetRawEncryptionKey should not expect that
the key is never empty.

Bug: 908431
Change-Id: Id9eece1bdf4492778c18178efc97380b800f5034
Reviewed-on: https://chromium-review.googlesource.com/c/1350915
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611249}

components/os_crypt/os_crypt_mac.mm

@@ -142,11 +142,12 @@ std::string OSCrypt::GetRawEncryptionKey() {
 
 // static
 void OSCrypt::SetRawEncryptionKey(const std::string& raw_key) {
-  DCHECK(!raw_key.empty());
   base::AutoLock auto_lock(g_lock.Get());
-  auto key = crypto::SymmetricKey::Import(crypto::SymmetricKey::AES, raw_key);
   DCHECK(!g_key_is_cached) << "Encryption key already set.";
-  g_cached_encryption_key = key.release();
+  if (!raw_key.empty()) {
+    auto key = crypto::SymmetricKey::Import(crypto::SymmetricKey::AES, raw_key);
+    g_cached_encryption_key = key.release();
+  }
   g_key_is_cached = true;
 }