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..a6defc68d853d95dbe9521528736af93099b2e9a 100644
--- a/chrome/browser/permissions/permission_context_base.cc
+++ b/chrome/browser/permissions/permission_context_base.cc
@@ -16,10 +16,18 @@
 #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"
 
+// API kill switch Finch Trials
+// static
+const char PermissionContextBase::kApiKillSwitchFieldStudy[] = "ApiKillSwitch";
+// static
+const char PermissionContextBase::kApiKillSwitchFieldParamEnabledValue[] =
+    "enabled";
+
 PermissionContextBase::PermissionContextBase(
     Profile* profile,
     const ContentSettingsType permission_type)
@@ -42,6 +50,15 @@ void PermissionContextBase::RequestPermission(
     const BrowserPermissionCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
+  // First check if this API has been disabled through Finch.
+  if (IsApiKillSwitchOn()) {
+    // The 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 +70,11 @@ void PermissionContextBase::RequestPermission(
 ContentSetting PermissionContextBase::GetPermissionStatus(
     const GURL& requesting_origin,
     const GURL& embedding_origin) const {
+
+  // If the API has been disabled through Finch, block all requests.
+  if (IsApiKillSwitchOn())
+    return CONTENT_SETTING_BLOCK;
+
   if (IsRestrictedToSecureOrigins() &&
       !content::IsOriginSecure(requesting_origin)) {
     return CONTENT_SETTING_BLOCK;
@@ -256,3 +278,11 @@ void PermissionContextBase::UpdateContentSetting(
       ContentSettingsPattern::FromURLNoWildcard(embedding_origin),
       permission_type_, std::string(), content_setting);
 }
+
+bool PermissionContextBase::IsApiKillSwitchOn() const {
+  const std::string param =
+      variations::GetVariationParamValue(kApiKillSwitchFieldStudy,
+          PermissionContextUmaUtil::GetPermissionString(permission_type_));

[mlamouri (slow - plz ping), 2015/10/12 10:51:10]

I guess I missed the PermissionContextUmaUtil::GetPermissionString() here. It
explains why you are adding this static method and how this is per-permission. I
would prefer to not have PermissionContextBase getting the string from
PermissionContextUmaUtil.

Maybe you could move that to a new file called permission_utils.{h,cc}?

[kcarattini, 2015/10/13 03:06:22]

I was thinking it would be better design to get rid of these switch statements
and add a 'std::string name'_ field to the base class, which would be
initialised int eh constructor of each subclass. What do you think? Would you be
ok with me leaving this cl as-is and doing this refactoring in a follow-up cl?

[raymes, 2015/10/13 03:34:08]

I like this idea :)

+
+  return param == kApiKillSwitchFieldParamEnabledValue;
+}