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.

Previous update was here:
#6097

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

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

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

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'),