Upgrades the ICU version to 64.2

The current version is 62.1, which is a now 3 major releases behind,
given that ICU 65 was released on Oct 3, 2019.

The version bump required an update to buildroot to fix too strict compilation
of ICU.

Previous update was here:
flutter#6097

I was advised to upgrade in the code review for this PR:
flutter#13045

This change brings in the function `icu::Locale::forLanguageTag(...)`
which is directly relevant to the work in the PR, but also updates
the library considerably.

* TODO(filmil): Release notes (will work with reviewers)

Tested:

```bash
#!/bin/bash

set -x

readonly FLUTTER_ENGINE_DIR="${FLUTTER_ENGINE_DIR:-$HOME/fx/flutter/engine/src}"
readonly OUT_DIR="${FLUTTER_ENGINE_DIR}/out"

(
  cd ${FLUTTER_ENGINE_DIR}

  ./flutter/tools/gn --unoptimized
  ninja -j 100 -C "${OUT_DIR}/host_debug_unopt"
  ./flutter/testing/run_tests.py --type engine
)
```

Author: filmil

DEPS

@@ -136,7 +136,7 @@ allowed_hosts = [
 ]
 
 deps = {
-  'src': 'https://github.com/flutter/buildroot.git' + '@' + 'edd0f2d19604a7d63deb20e9c19c1412acb8a9bd',
+  'src': 'https://github.com/flutter/buildroot.git' + '@' + '9c893948fd2413b32669b5a9261a1b6a487b8f9b',
 
    # Fuchsia compatibility
    #
@@ -177,7 +177,7 @@ deps = {
    Var('chromium_git') + '/chromium/src/ios.git' + '@' + Var('ios_tools_revision'),
 
   'src/third_party/icu':
-   Var('chromium_git') + '/chromium/deps/icu.git' + '@' + 'c56c671998902fcc4fc9ace88c83daa99f980793',
+   Var('chromium_git') + '/chromium/deps/icu.git' + '@' + '5005010d694e16571b8dfbf07d70817841f80a69',
 
   'src/third_party/boringssl':
    Var('github_git') + '/dart-lang/boringssl_gen.git' + '@' + Var('dart_boringssl_gen_rev'),