The UCRT is now optional and it failed to install on one builder.
Making optional to fix build break

If this doesn't fix the break on the waterfall then the switch
to VS 2015 will need to be reverted (crrev.com/1598493004).

TBR=scottmg@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1680433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298634 0039d316-1c4b-4281-b951-d872f2087c98