Have Dictionary use V8TRYCATCH_FOR_V8STRINGRESOURCE() macro

Have Dictionary use V8TRYCATCH_FOR_V8STRINGRESOURCE() macro instead of
the deprecated toWebCoreString().

R=haraken
BUG=

Review URL: https://chromiumcodereview.appspot.com/23812006

git-svn-id: svn://svn.chromium.org/blink/trunk@157541 bbb929c8-8fbe-4397-9dbb-9b2b20218538