Enable icu-timezone-data by default

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 in en-US locale:

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

New in German locale:

Thu Jun 22 2017 00:00:00 GMT-0700 (Nordamerikanische Westküsten-Sommerzeit)
Fri Dec 22 2017 00:00:00 GMT-0800 (Nordamerikanische Westküsten-Normalzeit)

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

Change-Id: I4e7fd8b3ddae5c7779e220c4c101e45904fcdc01
Reviewed-on: https://chromium-review.googlesource.com/625164
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47953}

src/flag-definitions.h

@@ -254,8 +254,7 @@ 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