WebRTC: Poll Chromium's SVN mirror instead of Google Code.
WebRTC: Poll Chromium's SVN mirror instead of Google Code.

When switching builders to recipes, the bots uses the Chromium
mirror by default instead of the Google Code Subversion URL we've
been using before. This causes problems since the builders are
being triggered by changes in the Google Code Subversion repo
and fails to sync from the mirror since that revision has not yet
reached the mirror.

Switching to polling the Chromium mirror instead should solve this.

TEST=Started a local master and checked the right SVN URL is polled.

Review URL: https://codereview.chromium.org/257593002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@265909 0039d316-1c4b-4281-b951-d872f2087c98

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