Make CalendarAstronomer public

This is necessary for Chrome OS to calculate sunset time.

Bug:chromium:726654
Test: With https://codereview.chromium.org/2887913004 patched in, 'gn
Change-Id: Icf6a8cd833f8afcd1a2d0ce874cf1c8c92fcba44
gen <out_dir> --check' does not complain.
TBR=afakhry@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/520323


Reviewed-by: Jungshik Shin <jshin@chromium.org>

BUILD.gn

@@ -118,7 +118,6 @@ component("icui18n") {
     "source/i18n/anytrans.cpp",
     "source/i18n/anytrans.h",
     "source/i18n/astro.cpp",
-    "source/i18n/astro.h",
     "source/i18n/basictz.cpp",
     "source/i18n/bocsu.cpp",
     "source/i18n/bocsu.h",
@@ -451,6 +450,8 @@ component("icui18n") {
   ]
   # ls source/i18n/unicode/*h | sort | sed 's/^\(.*\)$/    "\1",/'
   public = [
+    // crbug.com/726654: Make astro.h public until the upstream adds an API.
+    "source/i18n/astro.h",
     "source/i18n/unicode/alphaindex.h",
     "source/i18n/unicode/basictz.h",
     "source/i18n/unicode/calendar.h",