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..22f66f175ebe143d0dec1eec289e3efb8892cf8e 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 permissions kill switch.
+  static const char kPermissionsKillSwitchFieldStudy[];

[Bernhard Bauer, 2015/10/21 10:29:03]

Explain that this is public for testing?

[mlamouri (slow - plz ping), 2015/10/21 11:26:06]

Maybe this could move to permission_util?

[kcarattini, 2015/10/26 00:30:59]

Added comment about testing. See my comment about moving to permission util
below.

+
+  // The field trial param to enable the global permissions kill switch.
+  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.
@@ -79,6 +85,11 @@ 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.

[Bernhard Bauer, 2015/10/21 10:29:03]

It's a bit weird to put this into PermissionContextBase for permissions that
don't actually use it. Would it make more sense to put it into a class used by
either? Also, on which instance of PermissionContextBase would camera and
microphone call this method anyway?

[mlamouri (slow - plz ping), 2015/10/21 11:26:06]

I wouldn't mind having this moved to permission_util. It would only require to
take a ContentSettingsType.

[kcarattini, 2015/10/21 11:42:47]

Camera and Mic do use PermissionContextBase, they just don't use the
RequestPermission method at the moment. Right now they have a separate
implementation but there is every intention of refactoring it to use more of
PermissionContextBase like the other permissions. In the meantime, there is 
MediaStreamDevicePermissionContext used by MediaPermission that this method
could be called on.

[kcarattini, 2015/10/26 00:30:59]

I can see the reasoning for putting this in PermissionUtil. If we end up wanting
the kill switch for permissions that don't use PermissionContext then it makes
more sense to put it there. However, the discussions about the future of
permissions code are still ongoing and it's unclear to me how it will all turn
out. Since at the moment, the only permissions using the kill switch are in fact
PermissionContext permissions, it makes sense to me to keep it in the
PermissionContextBase class. If we do decide to expand the idea of permission
beyond PermissionContext then there will definitely be some code restructuring
going on and I can move it out then. How does that sound? 

+  bool IsPermissionKillSwitchOn() const;
+
  protected:
   // Decide whether the permission should be granted.
   // Calls PermissionDecided if permission can be decided non-interactively,