[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::OnContentScriptsExecuting(
    content::RenderFrameHost* host,
    const ScriptExecutionObserver::ExecutingScriptsMap& executing_scripts_map,
    const GURL& on_url) {
  FOR_EACH_OBSERVER(
      ScriptExecutionObserver,
      script_execution_observers_,
      OnScriptsExecuted(web_contents(), executing_scripts_map, on_url));
}

void TabHelper::OnContentScriptsExecuting(
    content::RenderFrameHost* host,
    const ScriptExecutionObserver::ExecutingScriptsMap& executing_scripts_map,
    const GURL& on_url) {
  FOR_EACH_OBSERVER(
      ScriptExecutionObserver,
      script_execution_observers_,
      OnScriptsExecuted(web_contents(), executing_scripts_map, on_url));
}
