Enable sync leveldb memory tweaks by default

The recently introduced feature toggle has been experimented with via
Finch, suggesting that the new settings cause an important improvement
in RAM usage (with early data, ~5 MB for the 50th percentile). In
addition, all sync status metrics report no regression, so let's enable
the change by default (but keep the kill switch around).

Bug: 887068
Change-Id: Ic3d8d1dff09900f0e3f25f75db4bce71572224cb
Reviewed-on: https://chromium-review.googlesource.com/c/1268117Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597552}

components/sync/model_impl/model_type_store_backend.cc

@@ -31,9 +31,9 @@ const char ModelTypeStoreBackend::kStoreInitResultHistogramName[] =
     "Sync.ModelTypeStoreInitResult";
 
 const base::Feature kModelTypeStoreAvoidReadCache{
-    "kModelTypeStoreAvoidReadCache", base::FEATURE_DISABLED_BY_DEFAULT};
+    "kModelTypeStoreAvoidReadCache", base::FEATURE_ENABLED_BY_DEFAULT};
 const base::Feature kModelTypeStoreSmallWriteBufferSize{
-    "kModelTypeStoreSmallWriteBufferSize", base::FEATURE_DISABLED_BY_DEFAULT};
+    "kModelTypeStoreSmallWriteBufferSize", base::FEATURE_ENABLED_BY_DEFAULT};
 
 namespace {