Adds a field trial to control Web API kill switches.

chrome/browser/permissions/permission_context_base.cc

Index: chrome/browser/permissions/permission_context_base.cc
diff --git a/chrome/browser/permissions/permission_context_base.cc b/chrome/browser/permissions/permission_context_base.cc
index f329b2bbb640a9b6826ee85bac39ff266dbc7528..69a0137e9d378aa154dc61cdeea76120925a5b37 100644
--- a/chrome/browser/permissions/permission_context_base.cc
+++ b/chrome/browser/permissions/permission_context_base.cc
@@ -16,10 +16,15 @@
 #include "chrome/common/pref_names.h"
 #include "components/content_settings/core/browser/host_content_settings_map.h"
 #include "components/content_settings/core/browser/website_settings_registry.h"
+#include "components/variations/variations_associated_data.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/common/origin_util.h"
 

[Nathan Parker, 2015/10/09 00:38:37]

Put in an anonymous namespace, or make static

[kcarattini, 2015/10/09 01:35:52]

Done.

+// API kill switch Finch Trials
+const char kApiFieldStudy[] = "ApiKillSwitch";
+const char kApiFieldParamEnabledValue[] = "enabled";
+
 PermissionContextBase::PermissionContextBase(
     Profile* profile,
     const ContentSettingsType permission_type)
@@ -42,6 +47,19 @@ void PermissionContextBase::RequestPermission(
     const BrowserPermissionCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
+  // First check if this API has been disabled through Finch.
+  const std::string param =
+      variations::GetVariationParamValue(kApiFieldStudy,

[Nathan Parker, 2015/10/09 00:38:37]

Meta: We should document how to use this kill switch.  I'm imagining an
emergency a year from now and people not knowing that this thing exists or how
to use it.

[Nathan Parker, 2015/10/09 00:38:37]

Since you use this twice, I'd make a helper func (in the anon-namespace) like
IsApiKillSwitchOn(permission_type) or some such.

[kcarattini, 2015/10/09 01:35:52]

Acknowledged.

[kcarattini, 2015/10/09 01:35:52]

Done.

+          PermissionContextUmaUtil::GetPermissionString(permission_type_));
+
+  if (param == kApiFieldParamEnabledValue) {
+    // This kill switch is enabled for this API; Block all requests.
+    NotifyPermissionSet(id, requesting_frame.GetOrigin(),
+        web_contents->GetLastCommittedURL().GetOrigin(), callback,
+        false /* persist */, CONTENT_SETTING_BLOCK);
+    return;
+  }
+
   DecidePermission(web_contents,
                    id,
                    requesting_frame.GetOrigin(),
@@ -53,6 +71,17 @@ void PermissionContextBase::RequestPermission(
 ContentSetting PermissionContextBase::GetPermissionStatus(
     const GURL& requesting_origin,
     const GURL& embedding_origin) const {
+
+  // First check if this API has been disabled through Finch.
+  const std::string param =
+      variations::GetVariationParamValue(kApiFieldStudy,
+          PermissionContextUmaUtil::GetPermissionString(permission_type_));
+
+  if (param == kApiFieldParamEnabledValue) {
+    // This kill switch is enabled for this API; Block all requests.
+    return CONTENT_SETTING_BLOCK;
+  }
+
   if (IsRestrictedToSecureOrigins() &&
       !content::IsOriginSecure(requesting_origin)) {
     return CONTENT_SETTING_BLOCK;