Electron fail to compile with system ICU

Since v0.37.0, Electron uses Chromium 49 which causes the final link of libchromiumcontent to fail with:

```
../../mojo/shell/public/cpp/initialize_base_and_icu.cc:40: error: undefined reference to 'base::i18n::InitializeICUFromRawMemory(unsigned char const*)'
```

For this reason we reverted to using bundled ICU in b1d42466dab81fcd19ba39cdd9f48b6ec3f8a797.
