Only fail in extract build if asked to do so.
Only fail in extract build if asked to do so.

Check first if halt_on_missing_build is present in
extract build.  This fixes a case where either a
slave is still using the dependent scheduler (which
can sometimes end up with the wrong revision number)
or a scheduler that's watching another tree (which may
not be the revision used in creating the build zip).

Review URL: http://codereview.chromium.org/6543026

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@75503 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build