V8 Buildbot: Allow overrides for uploading/downloading builds.
V8 Buildbot: Allow overrides for uploading/downloading builds.

This allows using these methods in the try recipe.

BUG=484589
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/1122443005

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@295133 0039d316-1c4b-4281-b951-d872f2087c98

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