[iOS] Adding ios_chromium_strings_grd and ios_google_chrome_strings_grd

Adding ios_chromium_strings_grd and ios_google_chrome_strings_grd directory
and OWNERS files. Those directories are used to store sha1 of screenshots,
for translation strings.
This is used by: crrev.com/c/1477231

Change-Id: Id3c17fde4e7234074bd22274e0918bd987682526
Reviewed-on: https://chromium-review.googlesource.com/c/1484306Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635087}

ios/chrome/app/strings/ios_chromium_strings_grd/OWNERS

+per-file *.sha1=*
+
+# TEAM: ios-directory-owners@chromium.org
+# OS: iOS

ios/chrome/app/strings/ios_google_chrome_strings_grd/OWNERS

+per-file *.sha1=*
+
+# TEAM: ios-directory-owners@chromium.org
+# OS: iOS