Don't clobber symlinks to build directories

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-Original-Commit-Position: refs/heads/master@{#444777}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 29acddfbe7b3c92cd85d99e416f920849d011ec1

clobber.py

@@ -37,6 +37,8 @@ def extract_gn_build_commands(build_ninja_file):
 
 
 def delete_dir(build_dir):
+  if os.path.islink(build_dir):
+    return
   # For unknown reasons (anti-virus?) rmtree of Chromium build directories
   # often fails on Windows.
   if sys.platform.startswith('win'):