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..b3a7963fcb00b3ff5411d29b65221b15657db74c 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 the global API kill switch.
+  static const char kApiKillSwitchFieldStudy[];
+
+  // The field trial param to enable the global API kill switch.
+  static const char kApiKillSwitchFieldParamBlockedValue[];

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

Could you have these two const char inside the .cc file instead of having them
static on the class?

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

They are here because they are used in the test as well. I also plan to add a
kill switch for camera/mic in another file.

+
   // 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;

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

I don't think this needs to be virtual.

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

Add a comment that this method is public for testing? (Or make it protected and
friend the test class.)

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

Done.

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

Done.

+
  protected:
   // Decide whether the permission should be granted.
   // Calls PermissionDecided if permission can be decided non-interactively,