Separate Server Lite Page from the rest of Previews browsertests.

No behaviorial changes, just moves the lite page tests to a separate
file.

Bug: 864635
Change-Id: I77a2b542febf1b7052085b401d3af13bbb41c825
Reviewed-on: https://chromium-review.googlesource.com/1213472
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589595}

chrome/test/BUILD.gn

@@ -726,6 +726,7 @@ test("browser_tests") {
       "../browser/prerender/prerender_test_utils.cc",
       "../browser/prerender/prerender_test_utils.h",
       "../browser/previews/previews_browsertest.cc",
+      "../browser/previews/previews_lite_page_browsertest.cc",
       "../browser/previews/previews_service_browser_test.cc",
       "../browser/previews/resource_loading_hints/resource_loading_hints_browsertest.cc",
       "../browser/process_singleton_browsertest.cc",