Re-enable icu-timezone-data by default

icu-timezone-data was enabled before but reverted due to a perf issue.
(sunspider/date-format-totfe regressed; crbug.com/769706 ).

However, my in-Chrome test of the same test [1] shows that there's virtually
no perf difference. See https://goo.gl/GX1jt6 .

This will introduce a new behavior on POSIX(-like) platforms. Timezone
names inside parentheses after GMT offset will not be 3-4 letter
abbreviation any longer. They'll be human-readable names in the current
default locale. This matches the current Windows behavior.

new Date(2017, 5, 22).toString()
new Date(2017, 11, 22).toString()

Current:

Thu Jun 22 2017 00:00:00 GMT-0700 (PDT)
Fri Dec 22 2017 00:00:00 GMT-0800 (PST)

New:

Thu Jun 22 2017 00:00:00 GMT-0700 (Pacific Daylight Time)
Fri Dec 22 2017 00:00:00 GMT-0800 (Pacific Standard Time)

This CL will be followed by
  https://chromium-review.googlesource.com/c/v8/v8/+/572148 to
implement https://github.com/tc39/ecma262/pull/778 .

[1] http://jungshik.github.io/v8/cr769706.html

BUG=v8:6031, v8:2137, v8:6076, chromium:769706
TEST=mjsunit/icu-date-lord-howe.js, mjsunit/icu-date-to-string.js

Change-Id: I22203670c3307a57fbf99e5f0a271dcbfbbef8fd
Reviewed-on: https://chromium-review.googlesource.com/857333
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51791}

src/flag-definitions.h

@@ -271,8 +271,7 @@ HARMONY_SHIPPING(FLAG_SHIPPING_FEATURES)
 #undef FLAG_SHIPPING_FEATURES
 
 #ifdef V8_INTL_SUPPORT
-DEFINE_BOOL(icu_timezone_data, false,
-            "get information about timezones from ICU")
+DEFINE_BOOL(icu_timezone_data, true, "get information about timezones from ICU")
 #endif
 
 #ifdef V8_ENABLE_FUTURE