To speed up Date.prototype.toString(), this patch adds a cache in
the DateCache for the string short name representing the time zone.
Because time zones in a particular location just have two short names
(for DST and standard time), and the DateCache already understands
whether a time is in DST or not, it is possible to keep the result
of OS::LocalTimezone around and select between the two based on
whether the time is DST or not.

In local microbenchmarks (calling Date.prototype.toString() in a
loop), I observed a 6-10% speedup with this patch. In the browser,
the speedup may be even greater as the system call needs to do
some extra work to break out of the sandbox. I don't think the
microbenchmark is extremely unrealistic; in any real program which
calls Date.prototype.toString() multiple times, the cache should
hit almost all of the time, as time zone changes are rare.

The proximate motivation for this patch was to enable ICU as a
backend for timezone information, which is drafted at
https://codereview.chromium.org/2724373002/
The ICU implementation of OS::LocalTimezone is even slower than
the system call one, but this patch makes their performance
indistinguishable on the microbenchmark.

In the tz database, many timezones actually do have a number of different
historical names. For example, America/Anchorage went through a number of
changes, from AST to AHST to YST to AKST. However, both ICU and the
Linux OS interfaces just report the modern timezone name in tests
for the appropriate timezone name, even for historical times. I can
see why this would be:
- For ICU, CLDR only has two short names in the data file: the one for
  dst and non-dst
- For Linux, the timezone names do seem to make it into the
  /etc/localtime file. However, glibc assumes there are only two relevant
  names and selects between them, as you can see in its implementation
  of localtime_r:
  http://bazaar.launchpad.net/~vcs-imports/glibc/master/view/head:/time/tzset.c#L573
So, this cache should be valid until we switch to a more accurate source
of short timezone names.

BUG=v8:6031

Review-Url: https://codereview.chromium.org/2726253002
Cr-Commit-Position: refs/heads/master@{#43730}