[Spellcheck] Enable the spelling service REST endpoint by default

The REST endpoint for the spelling service is ready to be fully launched.
It has been enabled at 1% of stable for 2 weeks now, and between control
and enabled on Dev/Canary/Beta/Stable:
- HTTP status code metrics have been stable, though we have noticed
  between 0.5% and 0.9% of requests give a 403. We will need to keep an
  eye on those, but they do not warrant delaying the endpoint migration.
- Proportion of failed requests has gone slightly down.
- Mean latency (ms) of requests has gone slightly down.

For those reasons, we are ready to fully migrate the spelling service to
the new REST endpoint. Code cleanup will follow shortly, once we've
confirmed that things are stable.

Bug: 943108
Change-Id: I19d394bed58db178dde682ffe168e651ee9e5125
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724666Reviewed-by: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: sebsg <sebsg@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#682385}

chrome/browser/spellchecker/spelling_service_client_unittest.cc

@@ -157,6 +157,9 @@ TEST_P(SpellingServiceClientTest, RequestTextCheck) {
   if (is_rest) {
     scoped_feature_list_.InitAndEnableFeature(
         spellcheck::kSpellingServiceRestApi);
+  } else {
+    scoped_feature_list_.InitAndDisableFeature(
+        spellcheck::kSpellingServiceRestApi);
   }
 
   PrefService* pref = profile_.GetPrefs();

components/spellcheck/common/spellcheck_features.cc

@@ -14,7 +14,7 @@ namespace spellcheck {
 #if BUILDFLAG(ENABLE_SPELLCHECK)
 
 const base::Feature kSpellingServiceRestApi{"SpellingServiceRestApi",
-                                            base::FEATURE_DISABLED_BY_DEFAULT};
+                                            base::FEATURE_ENABLED_BY_DEFAULT};
 
 #if defined(OS_WIN)
 const base::Feature kWinUseBrowserSpellChecker{