Background: in order to use multiple build directories without the
cognitive or scripting overhead of figuring out which one is currently
in use, I keep a symlink "out/cur" pointing to the current one.

When the clobber script runs, it detects that out/cur is a build
directory (because out/cur/args.gn exists) but shutil.rmtree() fails
when applied to a symlink. This change skips the clobber when a link is
detected and leaves out/cur pointing to the real build dir. There's no
need to remove the symlink, since the original has been clobbered.

Review-Url: https://codereview.chromium.org/2642833002
Cr-Commit-Position: refs/heads/master@{#444777}