V8 Buildbot: Raise stable timer for branches.
V8 Buildbot: Raise stable timer for branches.

The roll branches cause otherwise unwanted intermediate
builds each time a branch is created.

TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/1151313003

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@295376 0039d316-1c4b-4281-b951-d872f2087c98

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