Adds a minimal ICU data config for Flutter

This reduces the size of the ICU data file shipping with Flutter:
 from 6.3MB to 1.2MB uncompressed
 from 2.2MB to 534KB compressed (zip)

See go/flutter-icu for detailed analysis.

Change-Id: Iff5a4aa5286c685d5410b28d98ac8d49987d2eef
Reviewed-on: https://chromium-review.googlesource.com/1144234
Reviewed-by: Jungshik Shin <jshin@chromium.org>

Author: goderbauer

BUILD.gn

@@ -96,6 +96,8 @@ config("icu_code") {
       # certain preprocessor defines are set.
       # unistr.cpp also has an unused function for non-component builds.
       "-Wno-unused-function",
+      # putil.cpp contains unused variables when building for iOS simulators.
+      "-Wno-unused-variable",
     ]
   }
   if (is_clang || is_linux || is_android) {

README.chromium

@@ -77,7 +77,9 @@ Pre-built data files are generated and checked in with the following steps
 
      ix) Repeat ii) and iii) for iOS to get ios/icudtl.dat
 
-     x) scripts/clean_up_data_source.sh
+     x) Repeat ii) and iii) for Flutter to get flutter/icudtl.dat
+
+     xi) scripts/clean_up_data_source.sh
 
      This reverts the result of trim_data.sh and patch_locale.sh and
      make the tree ready for committing updated ICU data files for