Remove 'String::append' from 'Source/core/css/CSSComputedStyleDeclaration.cpp'.
Remove 'String::append' from 'Source/core/css/CSSComputedStyleDeclaration.cpp'.

BUG=268281

Review URL: https://chromiumcodereview.appspot.com/22148003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@155579 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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