Modifies the external extension preferences loader to load not only external_extensions.json but al…

This is based on a patch from Alexandre Abreu (http://codereview.chromium.org/9963120/) that I've modified slightly to work on Windows.

Modifies the external extension preferences loader to load not only external_extensions.json but also extensions based on standalone per-extension json files.

The new json files should be named <extension-id>.json and formatted as such:

{
  "external_crx": "<relative path from the .json file>",
  "external_version": "x.x"
}

Also, for Linux, added a new search path: /usr/share/[chromium|google-chrome]/extensions.

And finally, modifies the documentation to account for that change and to deprecate the external_extension.json files uses.

BUG=75174
TEST=None
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=134892

[Finnur, 2012-04-30 13:31:33]

Sam, can you take a look at the changes I did for
external_pref_extension_loader.cc (that's the only file that changed from Alex'
changelist, I believe).

[Alexandre Abreu, 2012-04-30 14:14:32]

On 2012/04/30 13:31:33, Finnur wrote:
> Sam, can you take a look at the changes I did for
> external_pref_extension_loader.cc (that's the only file that changed from
Alex'
> changelist, I believe).

Works for me as expected on Linux,

[skerner, 2012-04-30 18:26:48]

LGTM

There are uses of the external extensions json format other than the prefs, so
deprecating it will mean getting them to change their format.  Before actually
removing this, we need to talk to the providers of default apps and OEM ChromeOS
customization.

[Finnur, 2012-04-30 21:25:51]

I'm not in any hurry to remove external_extensions.json. Just wanted the new
scheme to be the de-facto cross-platform prefs standard. 

But, out of curiosity... Who uses external_extensions.json and for what purpose
other than the prefs system??

On 2012/04/30 18:26:48, skerner wrote:
> LGTM
> 
> There are uses of the external extensions json format other than the prefs, so
> deprecating it will mean getting them to change their format.  Before actually
> removing this, we need to talk to the providers of default apps and OEM
ChromeOS
> customization.

[Alexandre Abreu, 2012-05-15 15:31:54]

On 2012/04/30 21:25:51, Finnur wrote:
> I'm not in any hurry to remove external_extensions.json. Just wanted the new
> scheme to be the de-facto cross-platform prefs standard. 
> 
> But, out of curiosity... Who uses external_extensions.json and for what
purpose
> other than the prefs system??

Any news about that?

[Finnur, 2012-05-15 17:11:22]

On 2012/05/15 15:31:54, Alexandre Abreu wrote:
> On 2012/04/30 21:25:51, Finnur wrote:
> > I'm not in any hurry to remove external_extensions.json. Just wanted the new
> > scheme to be the de-facto cross-platform prefs standard. 
> > 
> > But, out of curiosity... Who uses external_extensions.json and for what
> purpose
> > other than the prefs system??
> 
> Any news about that?

Seems like a couple of internal projects within Chrome may have used it at some
point. Not clear whether they still do. I wouldn't worry too much about this,
since it will only affect them when we remove support. That's long term though,
no need to worry about it in the interim.