Follow-up to r19845 which suppresses syntax errors in presence of a stack overflow.

ReportUnexpectedToken already calls Traits::ReportMessageAt. If we're in Parser,
that already suppresses the syntax error. If we're in PreParser, we don't need
to suppress the syntax error (preparser errors don't go through Isolate, and
having both stack overflow and a syntax error present is handled correctly by
PreParserApi::PreParse).

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/197293003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19851 ce2b1a6d-e550-0410-aec6-3dcde31c8c00