Address a few TODOs in the gn config

Address a few TODOs in the gn config

- add direct dependant settings
- only compile mksnapshot on the host toolset

BUG=none
LOG=n
R=machenbach@chromium.org, brettw@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=21689

[Michael Achenbach, 2014-06-05 08:24:34]

lgtm (rubberstamp) - have too little understanding for a real lgtm

https://codereview.chromium.org/314073002/diff/1/BUILD.gn
File BUILD.gn (right):

https://codereview.chromium.org/314073002/diff/1/BUILD.gn#newcode975
BUILD.gn:975: if (current_toolchain == host_toolchain) {
nit: no indentation on that level is normal?

[jochen (gone - plz use gerrit), 2014-06-05 08:45:31]

Committed patchset #1 manually as r21689 (presubmit successful).