[Mac] Improve string use in PasteboardMac
https://bugs.webkit.org/show_bug.cgi?id=116418

Reviewed by Sam Weinig.

Did a Ben Poulain all over this file.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::plainText): Add the newline separately to the string builder.
Prepending it to the string first just does more allocations for no good reason.
(WebCore::Pasteboard::documentFragment): Use emptyString() instead of "" and use
ASCIILiteral where appropriate.
(WebCore::cocoaTypeFromHTMLClipboardType): More ASCIILiteral.
(WebCore::Pasteboard::clear): Use early return instead of a nested if here.
Use emptyString() instead of "".
(WebCore::addHTMLClipboardTypesForCocoaType): More ASCIILiteral.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150358 268f45cc-cd09-0410-ab3c-d52691b4dbfc