[Extensions] Rework inline installation observation

Instead of observing through the WebstoreAPI, observe directly in the TabHelper.
This is a great deal less code, more direct, and also fixes a lifetime issue
with the TabHelper being deleted before the inline installation completes.

BUG=613949

Review-Url: https://codereview.chromium.org/2103663002
Cr-Commit-Position: refs/heads/master@{#403188}
void TabHelper::DidCloneToNewWebContents(WebContents* old_web_contents,
                                         WebContents* new_web_contents) {
  CreateForWebContents(new_web_contents);
  TabHelper* new_helper = FromWebContents(new_web_contents);

  new_helper->SetExtensionApp(extension_app());
  new_helper->extension_app_icon_ = extension_app_icon_;
}

void TabHelper::DidCloneToNewWebContents(WebContents* old_web_contents,
                                         WebContents* new_web_contents) {
  CreateForWebContents(new_web_contents);
  TabHelper* new_helper = FromWebContents(new_web_contents);

  new_helper->SetExtensionApp(extension_app());
  new_helper->extension_app_icon_ = extension_app_icon_;
}
