Adds a command line flag --platform to control which platforms the given download occurs on.

Automatically marks downloads executable on non-Windows if the header x-goog-meta-executable is set. Automatically set this flag on upload when the executable bit is set.

BUG=233100

Review URL: https://codereview.chromium.org/42273002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@231387 0039d316-1c4b-4281-b951-d872f2087c98