Cleanup use of return value in StringCache

R=jochen@chromium.org
BUG=

ReturnValue is intended to be passed by value into any function that's not inlined or doesn't need access to stuff in FunctionCallbackInfo or PropertyCallbackInfo.

Review URL: https://chromiumcodereview.appspot.com/21022012

git-svn-id: svn://svn.chromium.org/blink/trunk@155319 bbb929c8-8fbe-4397-9dbb-9b2b20218538