Add comment to base::Feature::default_state

Emphasizing that it is not necessarily the actual state at runtime.
For example, a Finch experiment can be at 100%, so a feature can be
always-on despite being DISABLED_BY_DEFAULT.

Bug: none
Change-Id: I6dfaa86f4308f4c3f406c67d812c7ce7f5c0df22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1804458
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696975}

base/feature_list.h

@@ -24,6 +24,8 @@ class FieldTrial;
 class FieldTrialList;
 
 // Specifies whether a given feature is enabled or disabled by default.
+// NOTE: The actual runtime state may be different, due to a field trial or a
+// command line switch.
 enum FeatureState {
   FEATURE_DISABLED_BY_DEFAULT,
   FEATURE_ENABLED_BY_DEFAULT,
@@ -42,6 +44,8 @@ struct BASE_EXPORT Feature {
   const char* const name;
 
   // The default state (i.e. enabled or disabled) for this feature.
+  // NOTE: The actual runtime state may be different, due to a field trial or a
+  // command line switch.
   const FeatureState default_state;
 };