[webkitpy] Merge ChromiumBuildBot class into the BuildBot class.

Move the ChromiumBuildBot class logic into the BuildBot class.
Also remove some workarounds for detecting if the builder is a
Chromium builder.

BUG=none

Review URL: https://codereview.chromium.org/638573002

git-svn-id: svn://svn.chromium.org/blink/trunk@183408 bbb929c8-8fbe-4397-9dbb-9b2b20218538