Add ExtensionsApiClient interface, use it in the storage API SettingsFrontend

chrome/browser/extensions/chrome_extensions_browser_client.h

Index: chrome/browser/extensions/chrome_extensions_browser_client.h
diff --git a/chrome/browser/extensions/chrome_extensions_browser_client.h b/chrome/browser/extensions/chrome_extensions_browser_client.h
index d74c1cdba55efa25ff9832543affa69897642b4d..ea9155c6e7e95c07768ecb4d853a3cda5e217c30 100644
--- a/chrome/browser/extensions/chrome_extensions_browser_client.h
+++ b/chrome/browser/extensions/chrome_extensions_browser_client.h
@@ -20,6 +20,8 @@ class BrowserContext;
 
 namespace extensions {
 
+class ChromeExtensionsApiClient;
+
 // Implementation of extensions::BrowserClient for Chrome, which includes
 // knowledge of Profiles, BrowserContexts and incognito.
 //
@@ -76,6 +78,9 @@ class ChromeExtensionsBrowserClient : public ExtensionsBrowserClient {
   // Observer for Chrome-specific notifications.
   ChromeNotificationObserver notification_observer_;
 
+  // Client for API implementations.
+  scoped_ptr<ChromeExtensionsApiClient> api_client_;

[Devlin, 2014/03/03 17:07:59]

Would it make sense to put this in ExtensionsBrowserClient (since it will
presumably be in all implementations)?

[James Cook, 2014/03/03 19:53:09]

Unless you feel strongly I'd like to keep it here. It seems like we can either
have ExtensionsBrowserClient take care of Set(instance) and Set(NULL) in its
constructor/destructor (which SGTM) or we can have a public Set(foo) method that
takes ownership of |foo|.

Maybe there's some way to accomplish both but it seems like it would be ornate
and I would prefer the former.

[Devlin, 2014/03/03 20:25:27]

I don't feel strongly at all, so this is fine.  But, for what it's worth, all I
was thinking was:
class ExtensionsBrowserClient {
ExtensionsBrowserClient(ExtensionsApiClient* client) : api_client_(client) {}
...
scoped_ptr<ExtensionsApiClient> api_client_;
};

ChromeExtensionsBrowserClient::ChromeExtensionsBrowserClient()
    : ExtensionsBrowserClient(new ChromeExtensionsApiClient) {...}

ShellExtensionsBrowserClient::ShellExtensionsBrowserClient()
    : ExtensionsBrowserClient(new ShellExtensionsApiClient) {...}

which doesn't seem overlay ornate.  But again, I'm fine with this.

[Yoyo Zhou, 2014/03/04 19:55:04]

Do we want to be able to handle ExtensionsApiClient::Get() ever returning NULL?

+
   DISALLOW_COPY_AND_ASSIGN(ChromeExtensionsBrowserClient);
 };