Cached timestamps for toolchain update script

When cached timestamps are available, and toolchain is up-to-date,
reduces the runtime to ~.2s.

R=maruel@chromium.org
TBR=cpu@chromium.org
BUG=323300

Review URL: https://codereview.chromium.org/95983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238217 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed