Add set_pem_path() for ChromeTestExtensionLoader

The |pem_path_| member of ChromeTestExtensionLoader is currently always
empty and so will be unused. This CL adds a method to set the pem_path
to be used when packing an extension.

Change-Id: I977c29de7f8078d054ee9d8052c7628551356d1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332613
Commit-Queue: Jit Yao Yap <jityao@google.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797150}

chrome/browser/extensions/chrome_test_extension_loader.h

@@ -80,6 +80,7 @@ class ChromeTestExtensionLoader {
   void set_wait_for_renderers(bool wait_for_renderers) {
     wait_for_renderers_ = wait_for_renderers;
   }
+  void set_pem_path(const base::FilePath& pem_path) { pem_path_ = pem_path; }
 
  private:
   // Packs the extension at |unpacked_path| and returns the path to the created