Fix memory leak. Make "V8 Linux - memcheck" build bot happy.

Flags::SetFlagsFromCommandLine allocates memory to hold copies of string
arguments when calling Flag::set_string_value(..., true) and presently
noone deallocates this. Resetting the flags will clear this memory.
Since Flags can be used throughout the entire V8 lifetime,
Dispose/Teardown seems like the right location to free this
memory.

This is rarely a problem in practice, but the memcheck buildbot (rightly)
complains.

BUG=
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/505103003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23395 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
[Auto-roll] Bump up version to 3.29.18.0

TBR=machenbach@chromium.org

Review URL: https://codereview.chromium.org/502973006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23396 ce2b1a6d-e550-0410-aec6-3dcde31c8c00