Collapse symlinks when running "isolate.py remap".

Without this, "mb.py zip" can't include CIPD dependencies, which are
symlinks in the source tree.

Uses kjlubick's earlier work from
https://codereview.chromium.org/2844063005 .

Bug: 850107
Change-Id: Id96b85946ee15bcb2edfcf002016fcaad7b1a98b
Reviewed-on: https://chromium-review.googlesource.com/c/1338238Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608556}

tools/mb/mb.py

@@ -368,6 +368,7 @@ class MetaBuildWrapper(object):
             self.PathJoin(self.chromium_src_dir, 'tools', 'swarming_client',
                           'isolate.py'),
             'remap',
+            '--collapse_symlinks',
             '-s', self.PathJoin(self.args.path, self.args.target + '.isolated'),
             '-o', zip_dir
           ]