android: Use use_goma=1 gomadir gyp defines instead of magic GOMA_DIR env var.

For a short transition period, let envsetup.sh set the right GYP_DEFINES when
GOMA_DIR is set, so that people aren't broken immediately after this lands.

Also stop setting GOMA_COMPILER_PROXY_THREADS – if the value this sets
is truly better, that should be the default in goma, it shouldn't be set in
envsetup.sh (doing it in goma has the advantage that it works on all
platforms, also abstraction barriers, etc.)

BUG=332697
R=brettw@chromium.org, ilevy@chromium.org

Review URL: https://codereview.chromium.org/142393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245801