Ensure that swarming bootstrap always uses E drive even if called from cmd prompt.
Ensure that swarming bootstrap always uses E drive even if called from cmd prompt.

Review URL: https://codereview.chromium.org/461143002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@289086 0039d316-1c4b-4281-b951-d872f2087c98

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