StringBuilder::append should not use reserveCapacity.

reserveCapacity allocates the exact capacity specified which means
when we had a 16bit buffer in StringBuilder and were appending 8bit
strings to it we were constantly reallocating and copying the buffer.

Instead we can just call Vector::append() directly since it has a
template overload that can accept implicitly convertible types.

I noticed this when looking at lever.co's web app loading in
Instruments, it was spending 15% of the main thread time inside
TextResource::decodedText which was 83% memmove, and 11% munmap.

Switching to Vector::append should restore the correct size
doubling behavior when appending LChars to a UChar StringBuilder
and make this much faster.

This was a regression from when I switched to using a Vector
inside StringBuilder in:
https://codereview.chromium.org/2046353002

Review-Url: https://codereview.chromium.org/2192293002
Cr-Commit-Position: refs/heads/master@{#408869}

third_party/WebKit/Source/wtf/text/StringBuilder.cpp

@@ -200,9 +200,7 @@ void StringBuilder::append(const LChar* characters, unsigned length)
 
     ensureBuffer16(length);
     m_string = String();
-    m_buffer16->reserveCapacity(m_buffer16->size() + length);
-    for (size_t i = 0; i < length; ++i)
-        m_buffer16->uncheckedAppend(characters[i]);
+    m_buffer16->append(characters, length);
     m_length += length;
 }