Previously, callers needed to get string via a mode-specific string id
(e.g., IDS_PRODUCT_NAME or IDS_SXS_SHORTCUT_NAME for Chrome's localized
shortcut name). This meant that each caller needed to branch on the mode
(either explicitly, or implicitly via BrowserDistribution subclasses).

This change specifies mode-specific mappings for strings in
create_string_rc.py and plubing in GetLocalizedString so that callers
automatically get the proper string at runtime based on the mode at
play.

As a consequence, the following cleanups in this CL became possible:
- GoogleChromeSxSDistribution is removed, as it is no longer serves any
  purpose.
- InstallUtil::IsChromeSxSProcess is removed, as it no longer has any
  consumers.
- I took the liberty of sorting the STRING_IDS array in create_string_rc
  by ID name.

BUG=373987

Review-Url: https://codereview.chromium.org/2791593002
Cr-Commit-Position: refs/heads/master@{#461430}