Add ICU message format support

Adopt and customize a ICU message format wrapper used at Google to meet
Chromium's need.

This will enable formatting of 'complex messages' requiring plural and/or
selector (e.g. gender or 'single vs multiple') support with more than one
parameters.

Besides, l10n_util::GetPluralStringF* is rewritten to use this API.

I'm also planning to use this API to add a similar support to Chromium's
JavaScript-based UI and extensions.

References:
MessageFormat specs:
  http://icu-project.org/apiref/icu4j/com/ibm/icu/text/MessageFormat.html
  http://icu-project.org/apiref/icu4c/classicu_1_1DecimalFormat.html#details
Examples:
  http://userguide.icu-project.org/formatparse/messages
  message_formatter_unittest.cc
  go/plurals inside Google.

BUG=481734
TEST=base_unittests --gtest_filter="MessageFormat*"

Review URL: https://codereview.chromium.org/1140153005

Cr-Commit-Position: refs/heads/master@{#342327}

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