Add enable-stale-while-revalidate back to about:flags

The stale-while-revalidate feature will be available in M49 and we would
like developers to be able to test it, so restore the flag to
about:flags.

Also update the translations slightly.

BUG=564517
TEST=manual

Review URL: https://codereview.chromium.org/1499663005

Cr-Commit-Position: refs/heads/master@{#364294}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 0068375..34aa2871 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -1050,6 +1050,13 @@
      IDS_FLAGS_TRACE_UPLOAD_URL_DESCRIPTION,
      kOsAll,
      MULTI_VALUE_TYPE(kTraceUploadURL)},
+#if !defined(OS_IOS)
+    {"enable-stale-while-revalidate",
+     IDS_FLAGS_ENABLE_STALE_WHILE_REVALIDATE_NAME,
+     IDS_FLAGS_ENABLE_STALE_WHILE_REVALIDATE_DESCRIPTION,
+     kOsAll,
+     SINGLE_VALUE_TYPE(switches::kEnableStaleWhileRevalidate)},
+#endif
     {"enable-suggestions-with-substring-match",
      IDS_FLAGS_SUGGESTIONS_WITH_SUB_STRING_MATCH_NAME,
      IDS_FLAGS_SUGGESTIONS_WITH_SUB_STRING_MATCH_DESCRIPTION,