Switch main waterfall swarming priority to be 25, since CQ uses 30.
Switch main waterfall swarming priority to be 25, since CQ uses 30.

BUG=414808,401096

Review URL: https://codereview.chromium.org/597173002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@292112 0039d316-1c4b-4281-b951-d872f2087c98

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