https://bugs.webkit.org/show_bug.cgi?id=95508

Reviewed by Benjamin Poulain.

This patch removes all the uses of WTF::String::operator+= from
WebCore, except those in WorkerScriptLoader (which need a more delicate
patch). There were actually a handful of legitimate use cases for += in
this group. I've replaced them with calls to String::append rather than
+= so that we can remove += and encourage most contributors to use
more efficient string idioms.

(There are likely some more uses in WebCore hiding in port-specific
code---this patch covers only those call sites found when compiling the
chromium-mac port.)

* inspector/InspectorStyleTextEditor.cpp:
(WebCore::InspectorStyleTextEditor::insertProperty):
    - This complicated function looks really inefficient, but I didn't
      have the heart to rewrite it.
* inspector/NetworkResourcesData.cpp:
(WebCore::NetworkResourcesData::ResourceData::decodeDataToContent):
* loader/cache/CachedCSSStyleSheet.cpp:
(WebCore::CachedCSSStyleSheet::sheetText):
(WebCore::CachedCSSStyleSheet::data):
* loader/cache/CachedFont.cpp:
(WebCore::CachedFont::ensureSVGFontData):
* loader/cache/CachedScript.cpp:
(WebCore::CachedScript::script):
* loader/cache/CachedXSLStyleSheet.cpp:
(WebCore::CachedXSLStyleSheet::data):
    - This decoder/flush pattern can probably be improved by combining
      the decode and flush operations, but I didn't do that in this
      patch.
* page/FrameTree.cpp:
(WebCore::FrameTree::uniqueChildName):
    - I found this code very amusing. It's worried enough about
      efficiency to give a big speech about why snprintf is safe, but
      then it implicitly performs a large number of mallocs and memcpy
      operations.
* page/Page.cpp:
(WebCore::Page::userStyleSheet):
* platform/chromium/support/WebHTTPLoadInfo.cpp:
(WebKit::addHeader):
* platform/chromium/support/WebURLResponse.cpp:
(WebKit::WebURLResponse::addHTTPHeaderField):
    - This header-appending idiom looks like a reasonable use case for
      String::append.
* xml/XMLHttpRequest.cpp:
(WebCore::XMLHttpRequest::send):
(WebCore::XMLHttpRequest::setRequestHeaderInternal):
* xml/XPathFunctions.cpp:
(WebCore::XPath::FunTranslate::evaluate):
    - Fixes 6 year old FIXME.
* xml/parser/XMLDocumentParser.cpp:
(WebCore::XMLDocumentParser::append):
* xml/parser/XMLDocumentParser.h:
(XMLDocumentParser):
* xml/parser/XMLDocumentParserLibxml2.cpp:
(WebCore::XMLDocumentParser::doEnd):
* xml/parser/XMLDocumentParserQt.cpp:
(WebCore::XMLDocumentParser::doEnd):
    - Changed m_originalSourceForTransform to a SegmentedString so that
      we don't need to malloc and copy the entire document every time
      we get more data from the network.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@127366 268f45cc-cd09-0410-ab3c-d52691b4dbfc