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This change ensures that all English locale pak files (i.e.
en-US.pak and en-GB.pak) are stored inside an app bundle into
a directory named:

  assets/fallback-locales/

This ensures that said .pak files are always stored in the base
module split APK, and thus available on all Chrome installs, after
any system locale change.

This prevents Chrome from crashing on startup under rare
circumstances (see related bug for details).

Testing was performed manually because we don't have a good regression
test suite that can change the device's system locale programmatically
(see http://crbug.com/904360 for this). Following these steps:

1) Set device's language to Spanish
2) Ensure Chrome is not installed, then install the bundle.
3) Start Chrome. It displays its UI in Spanish.
4) Change system language to Slovenčina, and *remove* Spanish from
   the list of system languages.
5) Start Chrome. It displays its UI in English (the fallback)
   instead of crashing.

English is chosen because it is the fallback language that the Android
framework actually choses when the application doesn't have any
resources compatible with the current system language.

BUG=905725
R=agrieve@chromium.org,estevenson@chromium.org,yfriedman@chromium.org,tiborg@chromium.org,tedchoc@chromium.org

Change-Id: I2c1f53a805122b6e48fb5105c1d9aa8c6262fa1d
Reviewed-on: https://chromium-review.googlesource.com/c/1340303Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Eric Stevenson <estevenson@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: David Turner <digit@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609385}