[Cleanup] Switch from 'fetcher' to 'loader' naming in one_google_bar_*_unittest

This is a follow up of https://crrev.com/c/1043285, which completed the rest of
the migration from 'fetcher' to 'loader' terminology in
c/b/search/one_google_bar.

Bug: 773295
Change-Id: I77b875ffc64f7ce16ea4dfb265bbd73cd92cea0c
Reviewed-on: https://chromium-review.googlesource.com/1068023
Commit-Queue: Ramya Nagarajan <ramyan@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560678}

chrome/test/BUILD.gn

@@ -2959,7 +2959,7 @@ test("unit_tests") {
       "../browser/search/instant_service_unittest.cc",
       "../browser/search/instant_unittest_base.cc",
       "../browser/search/instant_unittest_base.h",
-      "../browser/search/one_google_bar/one_google_bar_fetcher_impl_unittest.cc",
+      "../browser/search/one_google_bar/one_google_bar_loader_impl_unittest.cc",
       "../browser/search/one_google_bar/one_google_bar_service_unittest.cc",
       "../browser/search/search_engine_base_url_tracker_unittest.cc",
       "../browser/search/search_unittest.cc",