Add PluginServiceStub for builds with enable_plugins==0

Add PluginServiceStub for builds with enable_plugins==0

BUG=162667

[nilesh, 2012-12-11 17:42:35]

Hi Sky/Brett, Please take a look.

Using PluginService interface in both chrome and content makes it easier to have
a stub implementation.

sky: content/browser/*
brettw: content/*.gypi

[jam, 2012-12-11 17:45:50]

As discussed before, we don't want to have stub objects for android builds. This
creates more work for developers when new methods are added (i.e. to remember to
update the stubs). More importantly though, the interface now is the public API.
The internal implementation (PluginServiceImpl) is what content internally
calls. Now it can always cast to that interface. If you add another
implementation, then casting won't work. This means that internal methods would
have to be put on the public interface, which is not good.

[nilesh, 2012-12-15 01:12:24]

On 2012/12/11 17:45:50, John Abd-El-Malek wrote:
> As discussed before, we don't want to have stub objects for android builds.
This
> creates more work for developers when new methods are added (i.e. to remember
to
> update the stubs). More importantly though, the interface now is the public
API.
> The internal implementation (PluginServiceImpl) is what content internally
> calls. Now it can always cast to that interface. If you add another
> implementation, then casting won't work. This means that internal methods
would
> have to be put on the public interface, which is not good.

Hi John, 
Thanks for your comments. I am trying a different approach which does not get
rid of the PluginServiceImpl. Please take a look:
https://codereview.chromium.org/11577031/