[iOS] Add a repack steps to iOS upstream for unit tests

//ios unit tests will soon required access to packed resources (both theme
and strings) so add steps required to repack data.

Those steps only pack what is required by //ios unit tests and thus pack to
repack_ios to avoid collision with downstream packing steps. Eventually the
upstream and downstream repack steps will be merged.

This forks some steps from //chrome since //ios cannot depends on //chrome
and those steps are too specific to Chrome to be moved higher in //build.

BUG=429756,475514

Review URL: https://codereview.chromium.org/1164983004

Cr-Original-Commit-Position: refs/heads/master@{#333229}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 6248cdd125afa37ac0f81f5bfda9219d153fcf1e

gyp_chromium

@@ -32,6 +32,7 @@ sys.path.insert(1, os.path.join(chrome_src, 'android_webview', 'tools'))
 sys.path.insert(1, os.path.join(chrome_src, 'build', 'android', 'gyp'))
 sys.path.insert(1, os.path.join(chrome_src, 'chrome', 'tools', 'build'))
 sys.path.insert(1, os.path.join(chrome_src, 'chromecast', 'tools', 'build'))
+sys.path.insert(1, os.path.join(chrome_src, 'ios', 'chrome', 'tools', 'build'))
 sys.path.insert(1, os.path.join(chrome_src, 'native_client', 'build'))
 sys.path.insert(1, os.path.join(chrome_src, 'native_client_sdk', 'src',
     'build_tools'))