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..1f882cbf2190c30ad80cf71529eb69fa72d3259c 100644
--- a/chrome/browser/permissions/permission_context_base.cc
+++ b/chrome/browser/permissions/permission_context_base.cc
@@ -11,15 +11,24 @@
 #include "chrome/browser/permissions/permission_context_uma_util.h"
 #include "chrome/browser/permissions/permission_queue_controller.h"
 #include "chrome/browser/permissions/permission_request_id.h"
+#include "chrome/browser/permissions/permission_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/website_settings/permission_bubble_manager.h"
 #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"
 
+// static
+const char PermissionContextBase::kPermissionsKillSwitchFieldStudy[] =
+    "PermissionsKillSwitch";
+// static
+const char PermissionContextBase::kPermissionsKillSwitchBlockedValue[] =
+    "blocked";
+
 PermissionContextBase::PermissionContextBase(
     Profile* profile,
     const ContentSettingsType permission_type)
@@ -42,6 +51,15 @@ void PermissionContextBase::RequestPermission(
     const BrowserPermissionCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
+  // First check if this permission has been disabled through Finch.

[Bernhard Bauer, 2015/10/21 10:29:03]

s/ through Finch/ if you plan on controlling this via a different mechanism in
the future, otherwise the comment will just rot.

(Also, don't use the term "Finch" in code, as it's a -- not particularly secret,
but still internal -- code name. The public name is "field trials".

[kcarattini, 2015/10/26 00:30:59]

Done.

+  if (IsPermissionKillSwitchOn()) {
+    // The kill switch is enabled for this permission; 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 +71,11 @@ void PermissionContextBase::RequestPermission(
 ContentSetting PermissionContextBase::GetPermissionStatus(
     const GURL& requesting_origin,
     const GURL& embedding_origin) const {
+
+  // If the permission has been disabled through Finch, block all requests.
+  if (IsPermissionKillSwitchOn())
+    return CONTENT_SETTING_BLOCK;
+
   if (IsRestrictedToSecureOrigins() &&
       !content::IsOriginSecure(requesting_origin)) {
     return CONTENT_SETTING_BLOCK;
@@ -256,3 +279,11 @@ void PermissionContextBase::UpdateContentSetting(
       ContentSettingsPattern::FromURLNoWildcard(embedding_origin),
       permission_type_, std::string(), content_setting);
 }
+
+bool PermissionContextBase::IsPermissionKillSwitchOn() const {
+  const std::string param =
+      variations::GetVariationParamValue(kPermissionsKillSwitchFieldStudy,
+          PermissionUtil::GetPermissionString(permission_type_));
+
+  return param == kPermissionsKillSwitchBlockedValue;
+}