Introduce Position::operator bool()

This patch introduces |operator bool()| for |PositionTemplate<T>| for writing
code like: |if (Position position = ...) { ... }|.

Change-Id: Ifb9061cb8ac95c25f7db03293c17080dba3ebc43
Reviewed-on: https://chromium-review.googlesource.com/939202Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539533}

third_party/WebKit/Source/core/editing/Position.h

@@ -73,6 +73,8 @@ class CORE_TEMPLATE_CLASS_EXPORT PositionTemplate {
 
   PositionTemplate(const PositionTemplate&);
 
+  explicit operator bool() const { return IsNotNull(); }
+
   PositionAnchorType AnchorType() const { return anchor_type_; }
   bool IsAfterAnchor() const {
     return anchor_type_ == PositionAnchorType::kAfterAnchor;

third_party/WebKit/Source/core/editing/PositionTest.cpp

@@ -141,6 +141,12 @@ TEST_F(PositionTest, NodeAsRangeLastNodeShadow) {
   EXPECT_EQ(t1, PositionInFlatTree::AfterNode(*host).NodeAsRangeLastNode());
 }
 
+TEST_F(PositionTest, OperatorBool) {
+  SetBodyContent("foo");
+  EXPECT_FALSE(static_cast<bool>(Position()));
+  EXPECT_TRUE(static_cast<bool>(Position(GetDocument().body(), 0)));
+}
+
 TEST_F(PositionTest, ToPositionInFlatTreeWithActiveInsertionPoint) {
   const char* body_content = "<p id='host'>00<b id='one'>11</b>22</p>";
   const char* shadow_content =