Add support for branded strings to iOS build.

Add support for branded strings to iOS build.

Add two new files ios_{chromium,google_chrome}_strings.grd (and the empty
.xtb files) to ios/chrome/app/strings for branded strings (i.e. strings
that reference the product, see http://goo.gl/6614MQ).

Fix ios/chrome/tools/build/ios_repack_locales.py to pack those strings
when branding flag is set, and require the flag to be set (except when
just listing the output).

Pass the -b flag when calling out ios_repack_locales.py script and update
tools/gritsettings/resource_ids to give some room for branded iOS strings.

Rename ios/chrome/app/strings/ios_strings_resources.grd to ios_strings.grd
to be in line with how the other string localization files are named.

BUG=475514

Review URL: https://codereview.chromium.org/1310073004

Cr-Commit-Position: refs/heads/master@{#346630}