We fetch the plugin list from three places. Previously, each location had custom code to proxy the query to the file thread. This change moves the query to the PluginService, which then internally manages posting to the file thread and calling back. I experimented with some different approaches to handling the lifetimes of the requests and responses. The approach now is simple: - The PluginService plugin methods are called and respond on the IO thread. Two of the three consumers of the plugin lists are already on the IO thread, so they don't need any complicated thread handling. - None of the callers ever need to cancel their requests: one is a singleton, and the other two always wait (in terms of holding a ref on the object) for the requests to complete. This makes lifetime management a lot simpler than it would otherwise be. With this change in place, I can then look at refactoring the PluginService implementation on Linux to do more complicated plugin loading as needed in bug 17863. BUG=17863 Review URL: http://codereview.chromium.org/437069 git-svn-id: svn://svn.chromium.org/chrome/trunk/src@33344 0039d316-1c4b-4281-b951-d872f2087c98