A very common pattern for the users of this script is to have binaries in
three different folders, eg:
some/folder/win/bin.exe
some/folder/mac/bin
some/folder/linux/bin

Instead of using --platform to specify three different paths, we can recognize
this usage and pass in --auto_platform, and then --directory some/folder/ as the
target.

When enumerating files, it will match each individual file to see if they have
(win|mac|linux) as a parent folder name, and filter out non-matching platforms.

BUG=
TEST= src/third_party/clang_format/bin$ download_from_google_storage --auto_platform --no_auth -b chromium-clang-fomat -dr .
0> Downloading ./linux/clang-format...

Review URL: https://codereview.chromium.org/126893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@245643 0039d316-1c4b-4281-b951-d872f2087c98