[base] Add Value::ClearList()

This change that adds base::Value::ClearList() as a convenient way to
completely clear a list Value.

Bug: 646113
Change-Id: I83d2a1b8f51595ca0e70b88d66013ea600fd4b98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1949798Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721466}

base/values.cc

@@ -436,6 +436,11 @@ size_t Value::EraseListValue(const Value& val) {
   return EraseListValueIf([&val](const Value& other) { return val == other; });
 }
 
+void Value::ClearList() {
+  CHECK(is_list());
+  list_.clear();
+}
+
 Value* Value::FindKey(StringPiece key) {
   return const_cast<Value*>(as_const(*this).FindKey(key));
 }

base/values.h

@@ -236,6 +236,10 @@ class BASE_EXPORT Value {
     return old_size - list_.size();
   }
 
+  // Erases all Values from the list.
+  // Note: This CHECKs that type() is Type::LIST.
+  void ClearList();
+
   // |FindKey| looks up |key| in the underlying dictionary. If found, it returns
   // a pointer to the element. Otherwise it returns nullptr.
   // returned. Callers are expected to perform a check against null before using
@@ -785,7 +789,7 @@ class BASE_EXPORT ListValue : public Value {
   explicit ListValue(ListStorage&& in_list) noexcept;
 
   // Clears the contents of this ListValue
-  // DEPRECATED, use GetList()::clear() instead.
+  // DEPRECATED, use ClearList() instead.
   void Clear();
 
   // Returns the number of Values in this list.

base/values_unittest.cc

@@ -599,6 +599,21 @@ TEST(ValuesTest, EraseListValueIf) {
   EXPECT_EQ(0u, value.EraseListValueIf([](const auto& val) { return true; }));
 }
 
+TEST(ValuesTest, ClearList) {
+  ListValue value;
+  value.Append(1);
+  value.Append(2);
+  value.Append(3);
+  EXPECT_EQ(3u, value.GetList().size());
+
+  value.ClearList();
+  EXPECT_TRUE(value.GetList().empty());
+
+  // ClearList() should be idempotent.
+  value.ClearList();
+  EXPECT_TRUE(value.GetList().empty());
+}
+
 TEST(ValuesTest, FindKey) {
   Value::DictStorage storage;
   storage.emplace("foo", std::make_unique<Value>("bar"));