createPlainText(): avoid a 32k buffer allocation for empty ranges.

Source/core/editing/iterators/TextIterator.cpp

Index: Source/core/editing/iterators/TextIterator.cpp
diff --git a/Source/core/editing/iterators/TextIterator.cpp b/Source/core/editing/iterators/TextIterator.cpp
index 0c9aa7edc6b851be5426c5e81f8d33c49afe9bb8..8f602548718493f4e5b9a6f64db96e6ea9fd21b7 100644
--- a/Source/core/editing/iterators/TextIterator.cpp
+++ b/Source/core/editing/iterators/TextIterator.cpp
@@ -1093,16 +1093,19 @@ static String createPlainText(const EphemeralRangeTemplate<Strategy>& range, Tex
         return emptyString();
 
     TextIteratorAlgorithm<Strategy> it(range.startPosition(), range.endPosition(), behavior);
-    // The initial buffer size can be critical for performance: https://bugs.webkit.org/show_bug.cgi?id=81192
-    static const unsigned initialCapacity = 1 << 15;
 
     unsigned bufferLength = 0;
     StringBuilder builder;
-    builder.reserveCapacity(initialCapacity);
+    if (!it.atEnd()) {

[yosin_UTC9, 2015/08/26 01:23:21]

Can we use early return pattern?

if (it.atEnd())
    return emptyString();

for (...) {
 ...
}

[sof, 2015/08/26 06:32:21]

That looks better, switched to an early return.

+        // The initial buffer size can be critical for performance: https://bugs.webkit.org/show_bug.cgi?id=81192
+        static const unsigned initialCapacity = 1 << 15;
 
-    for (; !it.atEnd(); it.advance()) {
-        it.text().appendTextToStringBuilder(builder);
-        bufferLength += it.length();
+        builder.reserveCapacity(initialCapacity);
+
+        for (; !it.atEnd(); it.advance()) {
+            it.text().appendTextToStringBuilder(builder);
+            bufferLength += it.length();

[yosin_UTC9, 2015/08/26 01:23:21]

Optional: Do we need to sum result length up?
It seems we should change an implementation of StringBuilder::toString()
in another patch.

Background:
If we want to create new empty string by StringBuilder::toString(),
we should change an implementation of StringBuilder::toString().
It seems it always create new empty string.

Current:
 String toString()
    {
        shrinkToFit();
        if (m_string.isNull())
            reifyString();
        return m_string;
    }

Suggested:
String toString()
{
  if (isEmpty())
    return emptyString();
  ... copy of original ...
}

[sof, 2015/08/26 06:32:21]

Just testing for builder.isEmpty() here would suffice rather than reworking
toString(), I'd say.

+        }
     }
 
     if (!bufferLength)