deps: turn in std::string for ICU

- node and v8 did not call into std::string previously,
so that access was shut off.
- this fixes compilation for ICU 58.2 (backlevel) but may
be expressed in other versions also.

Fixes: #19151

PR-URL: #19624
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

tools/icu/icu-generic.gyp

@@ -35,9 +35,7 @@
           'UCONFIG_NO_REGULAR_EXPRESSIONS=1',
           'U_ENABLE_DYLOAD=0',
           'U_STATIC_IMPLEMENTATION=1',
-          # Don't need std::string in API.
-          # Also, problematic: <http://bugs.icu-project.org/trac/ticket/11333>
-          'U_HAVE_STD_STRING=0',
+          'U_HAVE_STD_STRING=1',
           # TODO(srl295): reenable following pending
           # https://code.google.com/p/v8/issues/detail?id=3345
           # (saves some space)