This CL computes the correct input and output in the copy_tree GN template by
modifying tools/copy_tree.py to perform a dry-run that lists the files to be copied.
Also, shutil.copytree uses shutil.copy2 to copy files and their metadata, but copying
the metadata appears to confuse ninja dependency tracking.
To do the dry run and to use shutil.copy instead of shutil.copy2, copy_tree.py now
uses its own CopyTree function instead of shutil.copytree.
R=whesse@google.com
Review-Url: https://codereview.chromium.org/2875893002 .
Ryan Macnak