Add Windows official build symbols to FILES.cfg.
Add Windows official build symbols to FILES.cfg.

Also add a 'filegroup' field to make it easier to extract subsets of
files, replacing a hack based on the 'archive' field value.

Part 1/3 to implement the new 'filegroup/symbols' handling.

TEST=python tools/build/scripts/common/archive_utils_unittest.py --realfiles --src-base=.

Review URL: https://chromiumcodereview.appspot.com/9960051

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@131530 0039d316-1c4b-4281-b951-d872f2087c98

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