Fix passing testfilter through properly to triggered builds.
Fix passing testfilter through properly to triggered builds.

Using WithProperties() flatten the property as string. 'testfilter' is a list,
so this is really bad as it breaks the TestFilterImpl function to filter test
build steps.

R=szager@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/10933134

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@157262 0039d316-1c4b-4281-b951-d872f2087c98

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