Improve accuracy of estimate_capacity.py
Improve accuracy of estimate_capacity.py

Now it takes into account source stamp time as opposed to
build start time, which more realistically models pending queues.

BUG=none

Review URL: https://codereview.chromium.org/814023002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@293448 0039d316-1c4b-4281-b951-d872f2087c98

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