Rename translate force trigger on english pref

This allows all users to get the new backoff behavior, even if the old
behavior has already caused backoff for them.

Bug: 840367
Change-Id: Ia36720b0d71c3a69758a408d092b339ed487b195
Reviewed-on: https://chromium-review.googlesource.com/1217748Reviewed-by: Michael Martis <martis@chromium.org>
Commit-Queue: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590293}

components/translate/core/browser/translate_prefs.cc

@@ -34,7 +34,7 @@
 namespace translate {
 
 const char kForceTriggerTranslateCount[] =
-    "translate_force_trigger_on_english_count";
+    "translate_force_trigger_on_english_count_for_backoff";
 const char TranslatePrefs::kPrefTranslateSiteBlacklistDeprecated[] =
     "translate_site_blacklist";
 const char TranslatePrefs::kPrefTranslateSiteBlacklistWithTime[] =