Make TextIterator::RangeLength() to return valid value when ending in ::first-letter

This patch changes |TextIteratorTextNodeHandler::HandleTextBox()| to handle
range ends at end of leading collapsed whitespace of first-letter part, e.g.
"<p>^ |(1) foo</p>" where P has ::first-letter.

Bug: 810623
Change-Id: I6888bf0cba305d33c193b6cd4c3f191c8dcf36bd
Reviewed-on: https://chromium-review.googlesource.com/910543
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536916}

third_party/WebKit/Source/core/editing/iterators/TextIteratorTest.cpp

@@ -514,10 +514,7 @@ TEST_P(ParameterizedTextIteratorTest, RangeLengthWithFirstLetter) {
   InsertStyleElement("p::first-letter {font-size:200%;}");
   // Expectation should be as same as |RangeLengthBasic|
   EXPECT_EQ(0, TestRangeLength("<p>^| (1) abc def</p>"));
-  // TODO(editing-dev): We should fix legacy layout tree version.
-  // See http://crbug.com/810623
-  EXPECT_EQ(LayoutNGEnabled() ? 0 : 8,
-            TestRangeLength("<p>^ |(1) abc def</p>"));
+  EXPECT_EQ(0, TestRangeLength("<p>^ |(1) abc def</p>"));
   EXPECT_EQ(1, TestRangeLength("<p>^ (|1) abc def</p>"));
   EXPECT_EQ(2, TestRangeLength("<p>^ (1|) abc def</p>"));
   EXPECT_EQ(3, TestRangeLength("<p>^ (1)| abc def</p>"));

third_party/WebKit/Source/core/editing/iterators/TextIteratorTextNodeHandler.cpp

@@ -465,6 +465,12 @@ void TextIteratorTextNodeHandler::HandleTextBox() {
         return;
       }
       // Advance and continue
+      if (run_start == run_end && run_end == end) {
+        // "<p>^ |(1) foo</p>" with ::first-letter reaches here.
+        // Where "^" is start of range and "|" is end of range.
+        offset_ = end_offset_;
+        return;
+      }
       text_box_ = next_text_box;
       if (layout_object->ContainsReversedText())
         ++sorted_text_boxes_position_;