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 6c084e179c48da4dae0045acb35b18fed494336c..2db397c744dc195b1ea8a22abd7bcd2607186184 100644
--- a/chrome/browser/permissions/permission_context_base.h
+++ b/chrome/browser/permissions/permission_context_base.h
@@ -58,6 +58,14 @@ class PermissionContextBase : public KeyedService {
                         const ContentSettingsType permission_type);
   ~PermissionContextBase() override;
 
+  // A field trial used to enable the global permissions kill switch.
+  // This is public for testing purposes.
+  static const char kPermissionsKillSwitchFieldStudy[];
+
+  // The field trial param to enable the global permissions kill switch.
+  // This is public for testing purposes.
+  static const char kPermissionsKillSwitchBlockedValue[];
+
   // 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.
@@ -81,6 +89,11 @@ class PermissionContextBase : public KeyedService {
   virtual void CancelPermissionRequest(content::WebContents* web_contents,
                                        const PermissionRequestID& id);
 
+  // Whether the kill switch has been enabled for this permission.
+  // public for permissions that do not use RequestPermission, like
+  // camera and microphone, and for testing.
+  bool IsPermissionKillSwitchOn() const;
+
  protected:
   // Decide whether the permission should be granted.
   // Calls PermissionDecided if permission can be decided non-interactively,