Add id_generator.GetNextId() extension binding.

There are several APIs that need to generate unique integer ids. 
Each of them were implementing a native bindings for this and 
the implementations were almost identical. Now they all use the 
new GetNextId().

Review URL: https://codereview.chromium.org/25139003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226160 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226160