When parsing something like x.} the following would happen:
 - Parsing "." the lexer would move to the FIELDS start condition
 - Parsing } the lexer wouldn't find any <FIELDS> rule matching
 - The parser would fall back to <*>. that was asserted unreachable.

The fix is to add a <FIELDS>. rule to catch bad field starts

BUG=angleproject:1352

Change-Id: I262d2b9ef5f7346c19ae5e19a173e24f40f2f600
Reviewed-on: https://chromium-review.googlesource.com/338222Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>