Correct gyp toolsets for all combinations of want_separate_host_toolset and use_system_icu.

This CL ensures following logics:
if want_separate_host_toolset == 0 and use_system_icu == 0
  don't use system icu
  build icu for target toolset

elif want_separate_host_toolset == 0 and use_system_icu == 1
  use system icu for target toolset
  don't build icu

elif want_separate_host_toolset == 1 and use_system_icu == 0
  don't use system icu
  build icu for host and target

elif want_separate_host_toolset == 1 and use_system_icu == 1
  use system icu for target toolset
  build icu for host toolset

All complexity is due to the condition: want_separate_host_toolset == 1 and use_system_icu == 1.
Although use_system_icu == 1, this condition requires to build icu for host toolset.

BUG=
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/124003004

git-svn-id: https://src.chromium.org/chrome/trunk/deps/third_party/icu46@244134 4ff67af0-8c30-449e-8e8b-ad334ec8d88c