[Chrome Driver] Remove support for legacy background page key

The "background_page" key in an extension's manifest is restricted
to manifest v1 extensions, which we are removing support for. Remove
the check in ChromeDriver that looks for this.

Bug: 816677
Change-Id: I6835fecc261a4dbf8d2dafec47e3e32acddf107c
Reviewed-on: https://chromium-review.googlesource.com/988135Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549251}

chrome/test/chromedriver/chrome_launcher.cc

@@ -633,7 +633,6 @@ Status GetExtensionBackgroundPage(const base::DictionaryValue* manifest,
   if (manifest->Get("background.scripts", &unused_value))
     bg_page_name = "_generated_background_page.html";
   manifest->GetString("background.page", &bg_page_name);
-  manifest->GetString("background_page", &bg_page_name);
   if (bg_page_name.empty() || !persistent)
     return Status(kOk);
   GURL baseUrl("chrome-extension://" + id + "/");