Fix a browser crash on opening 'chrome://voicesearch' in guest mode.

Navigating directly to chrome://voicesearch in guest mode is causing
a crash, since the HotwordService is NULL in the guest mode. This
patch adds a NULL check on HotwordService to fix the crash.

BUG=411784
TEST=With --new-profile-management enabled, try to open chrome://voicesearch
in guest mode.

Review URL: https://codereview.chromium.org/554603002

Cr-Commit-Position: refs/heads/master@{#294149}

chrome/browser/ui/webui/voicesearch_ui.cc

@@ -245,7 +245,7 @@ class VoiceSearchDomHandler : public WebUIMessageHandler {
     std::string audio_logging_enabled = "No";
     HotwordService* hotword_service =
         HotwordServiceFactory::GetForProfile(profile_);
-    if (hotword_service->IsOptedIntoAudioLogging())
+    if (hotword_service && hotword_service->IsOptedIntoAudioLogging())
       audio_logging_enabled = "Yes";
     AddPair(list, ASCIIToUTF16("Hotword Audio Logging Enabled"),
             ASCIIToUTF16(audio_logging_enabled));