Fix Mac build breakage related to ExceptionCode/ExceptionState

This is a followup to r155527 which failed to update one of the Mac
specific files (WebSubstringUtil.mm)

BUG=252233
TBR=abarth

Review URL: https://codereview.chromium.org/22204002

git-svn-id: svn://svn.chromium.org/blink/trunk@155528 bbb929c8-8fbe-4397-9dbb-9b2b20218538