Clean up unnecessary copies in translate_prefs.

This CL cleans up unnecessary copies of std::strings and other objects
in components/translate/core/browser/translate_prefs.cc, as well as some
miscellaneous cleanup in this file.

This CL does not change any functionality, and is only clean up.

Bug: 1109032
Change-Id: I930d64accbf14e0b4163728a8ce23c1028e3f531
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2321768
Commit-Queue: Scott Little <sclittle@chromium.org>
Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792850}

components/translate/core/browser/translate_prefs.h

@@ -102,7 +102,11 @@ class DenialTimeUpdate {
 // Preferences and Language Settings.
 struct TranslateLanguageInfo {
   TranslateLanguageInfo();
+
   TranslateLanguageInfo(const TranslateLanguageInfo&);
+  TranslateLanguageInfo(TranslateLanguageInfo&&) noexcept;
+  TranslateLanguageInfo& operator=(const TranslateLanguageInfo&);
+  TranslateLanguageInfo& operator=(TranslateLanguageInfo&&) noexcept;
 
   // This ISO code of the language.
   std::string code;
@@ -200,7 +204,7 @@ class TranslatePrefs {
   // skip those languages while rearranging the list.
   void RearrangeLanguage(const std::string& language,
                          RearrangeSpecifier where,
-                         const int offset,
+                         int offset,
                          const std::vector<std::string>& enabled_languages);
 
   // Sets the language order to the provided order.