untrusted.gypi: Factor out duplicated "--gomadir" option
untrusted.gypi: Factor out duplicated "--gomadir" option

This can be moved to "common_args".

I tested this by doing the following:

cd ..
./native_client/build/gyp_nacl native_client/build/all.gyp \
    --generator-output=out-gyp -D use_goma=1 -D gomadir=MYGOMADIR

I checked that the makefiles in out-gyp pass "--gomadir=MYGOMADIR"
before and after the change.

I also checked that there was no change when omitting "-D gomadir=..."
and when passing "-D use_goma=0".

BUG=none
TEST=see above

Review URL: https://codereview.chromium.org/583583006

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@13760 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client