Adds a field trial to control Web API kill switches.

chrome/browser/permissions/permission_context_base.h

Index: chrome/browser/permissions/permission_context_base.h
diff --git a/chrome/browser/permissions/permission_context_base.h b/chrome/browser/permissions/permission_context_base.h
index 440880b957d9a3d30d8f65c321d79fbea0e53921..2fa6fd427ff1047606413eeb3c353826360fc11a 100644
--- a/chrome/browser/permissions/permission_context_base.h
+++ b/chrome/browser/permissions/permission_context_base.h
@@ -56,6 +56,12 @@ class PermissionContextBase : public KeyedService {
                         const ContentSettingsType permission_type);
   ~PermissionContextBase() override;
 
+  // A field trial used to enable thw global API kill switch.

[raymes, 2015/10/13 01:30:18]

nit: thw->the

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

Done.

+  static const char kApiKillSwitchFieldStudy[];
+
+  // The field trial param to enable an API kill switch.
+  static const char kApiKillSwitchFieldParamEnabledValue[];

[raymes, 2015/10/13 01:30:18]

nit: you've referred to the kill switch in two different ways in the comments
which makes it sound a bit like they are two different things (the global API
kill switch vs. an API kill switch). Perhaps just refer to both as "an API kill
switch"? (or maybe I'm missing some more complexity here :)

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

I made them both "global. I want to distinguish this kill switch from the
origin-specific one that we;ll do for Safe Browsing.

+
   // The renderer is requesting permission to push messages.
   // When the answer to a permission request has been determined, |callback|
   // should be called with the result.
@@ -79,6 +85,9 @@ class PermissionContextBase : public KeyedService {
   virtual void CancelPermissionRequest(content::WebContents* web_contents,
                                        const PermissionRequestID& id);
 
+  // Whether the kill switch has been enabled for this API.
+  virtual bool IsApiKillSwitchOn() const;
+
  protected:
   // Decide whether the permission should be granted.
   // Calls PermissionDecided if permission can be decided non-interactively,