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..1f2726f255ce6b8230150d5ea01be98888b145c6 100644
--- a/chrome/browser/permissions/permission_context_base.cc
+++ b/chrome/browser/permissions/permission_context_base.cc
@@ -16,10 +16,17 @@
 #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::kApiKillSwitchFieldStudy[] = "ApiKillSwitch";

[mlamouri (slow - plz ping), 2015/10/20 13:35:45]

I would prefer if the name of the switch was more descriptive. What about
"PermissionsAutoDeny" ?

[kcarattini, 2015/10/21 03:39:37]

I switched to "Permission" from "Api" since that's more accurate. I think "kill
switch" is an accepted term for what we're doing here, though. Especially since
we plan to move to the Finch Kill Switch infrastructure which has the term in
its name.

+// static
+const char PermissionContextBase::kApiKillSwitchFieldParamBlockedValue[] =
+    "blocked";

[mlamouri (slow - plz ping), 2015/10/20 13:35:45]

I think we should just try to match the string "enabled".

[kcarattini, 2015/10/21 03:39:37]

I actually moved from "enabled" to "blocked" because Ben found it confusing to
have the term "enabled" used when an API is disabled by a kill switch. I think
we should go for clarity and stick with "blocked".

+
 PermissionContextBase::PermissionContextBase(
     Profile* profile,
     const ContentSettingsType permission_type)
@@ -42,6 +49,15 @@ void PermissionContextBase::RequestPermission(
     const BrowserPermissionCallback& callback) {
   DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
 
+  // First check if this API has been disabled through Finch.
+  if (IsApiKillSwitchOn()) {

[mlamouri (slow - plz ping), 2015/10/20 13:35:45]

In order to make sure this is always called, I think you should make
::RequestPermission() not virtual. It is calling ::DecidePermission() which is
already virtual anyway.

[Bernhard Bauer, 2015/10/20 14:27:17]

I don't think that works, as this method is overridden by
GeolocationPermissionContext.

(It does mean that you'll need to check the kill switch in that class as well
though.)

[mlamouri (slow - plz ping), 2015/10/20 14:32:28]

The logic in GeolocationPermissionContext::RequestPermission could probably move
to ::DecidePermission().

[kcarattini, 2015/10/21 03:39:37]

GeolocationPermissionContext calls the base class method so it still picks up
some of this functionality (albeit after the extensions check).

[kcarattini, 2015/10/21 03:39:37]

I'm happy to do that in a followup cl, and then I could make RequestPermission
not virtual.

+    // 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 +69,11 @@ 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 +277,11 @@ void PermissionContextBase::UpdateContentSetting(
       ContentSettingsPattern::FromURLNoWildcard(embedding_origin),
       permission_type_, std::string(), content_setting);
 }
+
+bool PermissionContextBase::IsApiKillSwitchOn() const {

[mlamouri (slow - plz ping), 2015/10/20 13:35:45]

nit: maybe rename: ShouldAutoDeny()?

[kcarattini, 2015/10/21 03:39:37]

Changed from Api to Permission.

+  const std::string param =
+      variations::GetVariationParamValue(kApiKillSwitchFieldStudy,
+          PermissionContextUmaUtil::GetPermissionString(permission_type_));
+
+  return param == kApiKillSwitchFieldParamBlockedValue;
+}