Clean up buildbot context tests, get browser tests re-enabled on trybots
Clean up buildbot context tests, get browser tests re-enabled on trybots

The change in http://codereview.chromium.org/7565022 did not have
the intended effect of restoring the browser tests on the trybots.
Having the same magic tests in more than one place is manifestly
error-prone.  Clean it up to set some context variables saying what
we mean.

BUG= none
TEST= trybots

R=bradnelson@google.com,ncbray@google.com

Review URL: http://codereview.chromium.org/7511012

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@6318 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client