Introduce dynamic ablation of AutocompleteProviders using AC dynamic trials.

chrome/browser/autocomplete/autocomplete_field_trial.cc

Index: chrome/browser/autocomplete/autocomplete_field_trial.cc
diff --git a/chrome/browser/autocomplete/autocomplete_field_trial.cc b/chrome/browser/autocomplete/autocomplete_field_trial.cc
index 08e5410129479853e04e27d4825e3c719939a836..12c3f001d0213b450b6a2492a13c9d107aed59f5 100644
--- a/chrome/browser/autocomplete/autocomplete_field_trial.cc
+++ b/chrome/browser/autocomplete/autocomplete_field_trial.cc
@@ -105,10 +105,10 @@ const base::FieldTrial::Probability
 
 // Field trial IDs.
 // Though they are not literally "const", they are set only once, in
-// Activate() below.
+// ActivateStaticTrials() below.
 
 // Whether the static field trials have been initialized by
-// ActivateStaticTrials method.
+// ActivateStaticTrials() method.
 bool static_field_trials_initialized = false;
 
 // Field trial ID for the disallow-inline History Quick Provider
@@ -137,8 +137,14 @@ int hqp_only_count_matches_at_word_boundaries_experiment_group = 0;
 // experiment group.
 int hqp_use_cursor_position_experiment_group = 0;
 
+// Concatenates the autocomplete dynamic field trial prefix with a field trial
+// ID to form a complete autocomplete field trial name.
+std::string DynamicFieldTrialName(int id) {
+  return base::StringPrintf("%s%d", kAutocompleteDynamicFieldTrialPrefix, id);
 }
 
+}  // namespace
+
 
 void AutocompleteFieldTrial::ActivateStaticTrials() {
   DCHECK(!static_field_trials_initialized);
@@ -261,11 +267,40 @@ void AutocompleteFieldTrial::ActivateStaticTrials() {
 }
 
 void AutocompleteFieldTrial::ActivateDynamicTrials() {
-  // Initialize all autocomplete dynamic field trials.
+  // Initialize all autocomplete dynamic field trials.  This method may be
+  // called multiple times.
+  for (int i = 0; i < kMaxAutocompleteDynamicFieldTrials; ++i)
+    base::FieldTrialList::FindValue(DynamicFieldTrialName(i));
+}
+
+int AutocompleteFieldTrial::GetDisabledProviderTypes() {
+  // Make sure that Autocomplete dynamic field trials are activated.  It's OK to
+  // call this method multiple times.
+  ActivateDynamicTrials();
+
+  // We assume that each group may contain a substring
+  // "DisabledProviders_<mask>" where "mask" is a bitmap of disabled provider
+  // types (AutocompleteProvider::Type).
+  int provider_types = 0;
   for (int i = 0; i < kMaxAutocompleteDynamicFieldTrials; ++i) {
-    base::FieldTrialList::FindValue(
-        base::StringPrintf("%s%d", kAutocompleteDynamicFieldTrialPrefix, i));
+    std::string group_name = base::FieldTrialList::FindFullName(
+        DynamicFieldTrialName(i));
+    if (!group_name.empty()) {
+      const char kDisabledProviders[] = "DisabledProviders_";
+      const size_t pos = group_name.find(kDisabledProviders);
+      if (pos == std::string::npos)
+        continue;
+      int types = 0;
+      // It's OK to ignore the result, because we don't care about perfect
+      // conversions.
+      base::StringToInt(base::StringPiece(
+          group_name.substr(pos + strlen(kDisabledProviders))), &types);
+      if (types == 0)
+        LOG(WARNING) << "Expecting a non-zero bitmap; group = " << group_name;
+      provider_types |= types;
+    }
   }
+  return provider_types;
 }
 
 bool AutocompleteFieldTrial::InDisallowInlineHQPFieldTrial() {