Command-line flags can be parsed in two modes. In the mode used by
Chrome, an unrecognized flag causes the remaining arguments to be
ignored. This is different from how d8 parses flags.

Example:

1) d8 --enable-slow-asserts --trace-ic
2) content_shell --js-flags='--enable-slow-asserts --trace-ic'

Assuming we compiled without ENABLE_SLOW_DCHECKS, in (1) we get a
warning that --enable-slow-asserts is unknown. Nevertheless,
--trace-ic will be enabled. In (2), we get an error that
--enable-slow-asserts is unknown but --trace-ic will NOT be enabled
(and neither does content_shell abort).

This inconsistency is obviously very confusing. With this CL, we
will at least print any flags that got ignored.

Change-Id: I22bdb06d2b0accc234b3f5d596458809de364bce
Reviewed-on: https://chromium-review.googlesource.com/1066010
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53261}