[Extensions] Enable sideloading prompt on mac on canary

BUG=669277

Review-Url: https://codereview.chromium.org/2623033002
Cr-Commit-Position: refs/heads/master@{#442983}

chrome/browser/extensions/extension_service.cc

@@ -2258,7 +2258,7 @@ int ExtensionService::GetDisableReasonsOnInstalled(const Extension* extension) {
                : disable_reasons;
   }
 
-  if (FeatureSwitch::prompt_for_external_extensions()->IsEnabled()) {
+  if (extensions::ExternalInstallManager::IsPromptingEnabled()) {
     // External extensions are initially disabled. We prompt the user before
     // enabling them. Hosted apps are excepted because they are not dangerous
     // (they need to be launched by the user anyway). We also don't prompt for

chrome/browser/extensions/external_install_manager.cc

@@ -84,6 +84,16 @@ ExternalInstallManager::ExternalInstallManager(
 ExternalInstallManager::~ExternalInstallManager() {
 }
 
+
+bool ExternalInstallManager::IsPromptingEnabled() {
+  // Enable this feature on canary on mac.
+#if defined(OS_MACOSX) && defined(GOOGLE_CHROME_BUILD)
+  return GetCurrentChannel() <= version_info::Channel::CANARY;
+#else
+  return FeatureSwitch::prompt_for_external_extensions()->IsEnabled();
+#endif
+}
+
 void ExternalInstallManager::AddExternalInstallError(const Extension* extension,
                                                      bool is_new_profile) {
   // Error already exists or has been previously shown.
@@ -116,7 +126,7 @@ void ExternalInstallManager::RemoveExternalInstallError(
 
 void ExternalInstallManager::UpdateExternalExtensionAlert() {
   // If the feature is not enabled do nothing.
-  if (!FeatureSwitch::prompt_for_external_extensions()->IsEnabled())
+  if (!IsPromptingEnabled())
     return;
 
   // Look for any extensions that were disabled because of being unacknowledged
@@ -219,7 +229,7 @@ void ExternalInstallManager::OnExtensionUninstalled(
 
 bool ExternalInstallManager::IsUnacknowledgedExternalExtension(
     const Extension& extension) const {
-  if (!FeatureSwitch::prompt_for_external_extensions()->IsEnabled())
+  if (!IsPromptingEnabled())
     return false;
 
   int disable_reasons = extension_prefs_->GetDisableReasons(extension.id());

chrome/browser/extensions/external_install_manager.h

@@ -34,6 +34,9 @@ class ExternalInstallManager : public ExtensionRegistryObserver,
                          bool is_first_run);
   ~ExternalInstallManager() override;
 
+  // Returns true if prompting for external extensions is enabled.
+  static bool IsPromptingEnabled();
+
   // Removes the error associated with a given extension.
   void RemoveExternalInstallError(const std::string& extension_id);